[log_writer_init] It is possible, that the writer's queue is NULL in the initialization time

It means, that the log_writer_init returns with FALSE, and all of the writer based destinations has to handle
if the initialization of writer is failed.

Signed-off-by: Juhasz Viktor <jviktor@balabit.hu>
Signed-off-by: Daniel Gados <gdani@balabit.hu>
logwriter: Implementing the late acknowledge mechanism on the writer's side

Signed-off-by: Daniel Gados <gdani@balabit.hu>
logqueue: refactoring pop_head and adding log_queue_rewind_backlog to log_writer_flush

Signed-off-by: Daniel Gados <gdani@balabit.hu>
logproto-client: Implemenation of the 'keep one message' functionality

Signed-off-by: Daniel Gados <gdani@balabit.hu>