Add SNMP datagram parsing support.

This supports parsing of SNMPv1 (RFC 1157), SNMPv2 (RFC 1901/3416), and
SNMPv2 (RFC 3412).  An event is raised for each SNMP PDU type, though
there's not currently any event handlers for them and not a default
snmp.log either.  However, simple presence of SNMP is currently visible
now in conn.log service field and known_services.log.
Fix compiler nitpicks from new SNMP code.
Add memory leak unit test for SNMP.
Improve __load__.bro creation for .bif.bro stubs.

The new loader scripts should not try to @load .bif.bro scripts that
were part of a build of an earlier Bro version, but have since had their
.bif file removed.
change the sumstats table that tracks recent intermediate requests
back to create interval.

In this instance a read expiry actually makes much less sense.
Merge remote-tracking branch 'origin/topic/jsiwek/bif-loader-scripts'

BIT-1175 #merged

* origin/topic/jsiwek/bif-loader-scripts:
  Improve __load__.bro creation for .bif.bro stubs.
Updating submodule(s).

 [nomail]
Merge remote-tracking branch 'origin/fastpath'

* origin/fastpath:
  change the sumstats table that tracks recent intermediate requests back to create interval.
Merge remote-tracking branch 'origin/topic/jsiwek/snmp'

* origin/topic/jsiwek/snmp:
  Add memory leak unit test for SNMP.
  Fix compiler nitpicks from new SNMP code.
  Add SNMP datagram parsing support.

BIT-1142
Updating test baselines.
Merge branch 'master' into topic/jsiwek/bit-348
Update SNMP analyzer's DeliverPacket method signature.