#227 Do not overwrite partially retrieved data with null or incorrect values

this and two related commits implements:
- setting snmp_field_protection which can be used to protect any
  database field from having a null value set
- extra device discover worker which prefetches SNMP interfaces and
  performs some sanity checks and comparison against the DB
the two checks will return job status ERROR with CANCELLED if they
see anything fishy.

share/config.yml

@@ -233,7 +233,7 @@ snmpretries: 2
 snmp_remoteport: {}
 snmp_field_protection:
   device:
-    serial: ['any']
+    serial: ['group:__ANY__']
 devices_no: []
 devices_only: []
 discover_no: []