logthrdestdrv: Fixes for centralized retries() functionality

New worker insert result type added: WORKER_INSERT_RESULT_NOT_CONNECTED.

When an insert() failes due to a connection error, syslog-ng
 - should rewind the message
 - should do suspend()
 - and should not increment retries counter.

Signed-off-by: Budai Laszlo <Laszlo.Budai@balabit.com>