logthrdestdrv: Move SEQNUM handling here

Instead of handling SEQNUM in all the various drivers that use
LogThreadedDestDriver, move the functionality into the class itself.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
logthrdestdrv: Implement SEQNUM persistence

With this change, LogThreadedDestDriver will store SEQNUM in the persist
file, so we won't start from one again after a reload or a restart. The
key used for storing is the driver-supplied persist_name plus ".seqnum"
appended.

We do the persisting at deinit, and the loading in driver start. We
can't do it at init time, because the formatting function is not set
there yet. Fortunately, we do not need the SEQNUM between init and
start, so we can do it at a later time.

This fixes #178.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #200 from algernon/f/3.6/logthr-seqnum

SEQNUM persistence for LogThreadedDestDriver modules