[log_pipe_init] Handling the result of log_pipe_init

From now the LogWriter returns FALSE from log_writer_init
if the queue is a null-pointer. The return value of log_pipe_init
is handled not only when it is called upon a LogWriter but for
any other LogPipe as well.

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>