mon/MonClient: do not send_log if conn is not active anymore

mon/MonClient: do not send_log if conn is not active anymore

the log message to be sent will be appended to waiting_for_session
instead. but we will send the logs anyway when the MonClient is
authorized with the new connection. so, avoid doing this as it's not
necessary.

also refactor the schedule_tick() call into a scope_guard, so it is
always called upon the return of the tick() method.

Signed-off-by: Kefu Chai <kchai@redhat.com>