Avoid lock/defer of jobs deined via ACL

This commit adds a table 'device_skip' that is used to restrict job queue
searches to avoid jobs that are not permitted on this backend via *_no ACLs,
or jobs on devices that have previously encountered multiple SNMP timeouts.

When the backend loads or a device is added, a row is added to the table if
that device should not be polled on this backend (together with the job
actions which are to be skipped/denied). When a device SNMP connect fails a
counter in the same row (or a new row) is incremented.

There is also a new report 'SNMP Connect Failures' to show the devices with
non-zero SNMP connect failure counters. A configurable limit in the setting
'max_deferrals' is used to set the threshold of no longer polling the device.

To reset the deferrals/failures count, restart the Netdisco backend (which
regenerates 'device_skip' cache entries).

Squashed commit of the following:

commit b5e32c219d
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 20:55:14 2017 +0100

    show all failed connections in report

commit ffce3cee84
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 20:12:39 2017 +0100

    only resolve fqdn once

commit cc4f680f01
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 20:10:20 2017 +0100

    Revert "only resolve fqdn once"

    This reverts commit 3d136a54de.

commit d8d082b30e
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 20:09:05 2017 +0100

    a report to show SNMP failures

commit 3d136a54de
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 19:37:58 2017 +0100

    only resolve fqdn once

commit 4550b8a84c
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 17:27:43 2017 +0100

    skipover now implicit from deferrals/actionset; fix sql where logic with better correlation

commit b51edbccd2
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 16:11:29 2017 +0100

    only abort lock if action matches badactions

commit 415559b24f
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 13:56:42 2017 +0100

    set skipover true when adding to actionset

commit 1086f2c467
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 13:50:56 2017 +0100

    fix empty actionset

commit 31962580b8
Merge: 9b2e993e 6808133b
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 13:25:08 2017 +0100

    Merge branch 'og-device_skip' of github.com:netdisco/netdisco into og-device_skip

commit 6808133bdb
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 13:19:54 2017 +0100

    in-job checks for acls are required for netdisco-do foreground actions

commit 3944dd7813
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 13:18:30 2017 +0100

    avoid extra device lookup

commit 9b2e993e0f
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 12:31:36 2017 +0100

    also delete device_skip rows when deleting device

commit b55854e91d
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 11:34:27 2017 +0100

    actions in device_skip table are now an array/set

commit 5e126eef07
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 09:36:33 2017 +0100

    typo

commit 44266f2767
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 09:14:25 2017 +0100

    *able checks within jobs should not be necessary with skiplist

commit e7c22e7d11
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 08:58:57 2017 +0100

    increment deferrals field when job is deferred

commit 88ae9c00ba
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 08:40:27 2017 +0100

    turn connect fail into defer

commit eac1857043
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Tue May 23 08:26:59 2017 +0100

    rename failures column to be deferrals

commit 96ed444bbb
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Mon May 22 22:52:51 2017 +0100

    set up list of jobs the backend instance should skip

commit 3a0019296d
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Mon May 22 22:01:50 2017 +0100

    separate out is_*able last_* checks

commit cf8589aba2
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Sun May 21 22:35:38 2017 +0100

    change from ignore to skip name

commit ed193356f8
Author: Oliver Gorwits <oliver@cpan.org>
Date:   Sun May 21 14:52:33 2017 +0100

    device_ignore table to track devices to skip in polling

lib/App/Netdisco/Backend/Worker/Poller/Device.pm

@@ -3,7 +3,7 @@ package App::Netdisco::Backend::Worker::Poller::Device;
 use Dancer qw/:moose :syntax :script/;
 
 use App::Netdisco::Util::SNMP 'snmp_connect';
-use App::Netdisco::Util::Device qw/get_device is_discoverable/;
+use App::Netdisco::Util::Device qw/get_device is_discoverable_now/;
 use App::Netdisco::Core::Discover ':all';
 use App::Netdisco::Backend::Util ':all';
 use App::Netdisco::JobQueue qw/jq_queued jq_insert/;
@@ -54,13 +54,13 @@ sub discover {
       return job_done("discover skipped: $host is pseudo-device");
   }
 
-  unless (is_discoverable($device->ip)) {
+  unless (is_discoverable_now($device)) {
       return job_defer("discover deferred: $host is not discoverable");
   }
 
   my $snmp = snmp_connect($device);
   if (!defined $snmp) {
-      return job_error("discover failed: could not SNMP connect to $host");
+      return job_defer("discover failed: could not SNMP connect to $host");
   }
 
   store_device($device, $snmp);