filter: Make fop_cmp_eval stateless

Filters can be used from multiple threads, do not store the internal
state within the FilterCmp object, because that has a race condition,
and may end up corrupting the state.

Signed-off-by: Andras Tim <tia@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>
syslog-format: Fix the NILVALUE handling

When handling a NILVALUE in the syslog timestamp, we should not do any
zone adjustments - there's none in the timestamp anyway.

Reported-by: Vincent Brillault <spam@lerya.net>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Preparations for 3.4.3

Signed-off-by: Gergely Nagy <algernon@balabit.hu>