Lock device_port table when updating Device Ports (closes #40)

Netdisco/lib/App/Netdisco/Core/Discover.pm

@@ -282,7 +282,7 @@ sub store_interfaces {
       };
   }
 
-  schema('netdisco')->txn_do(sub {
+  schema('netdisco')->resultset('DevicePort')->txn_do_locked(sub {
     my $gone = $device->ports->delete({keep_nodes => 1});
     debug sprintf ' [%s] interfaces - removed %s interfaces',
       $device->ip, $gone;

Netdisco/lib/App/Netdisco/DB/ResultSet/DevicePort.pm

@@ -4,6 +4,10 @@ use base 'DBIx::Class::ResultSet';
 use strict;
 use warnings FATAL => 'all';
 
+__PACKAGE__->load_components(qw/
+  +App::Netdisco::DB::ExplicitLocking
+/);
+
 =head1 ADDITIONAL METHODS
 
 =head2 with_times