common: reset last_log_sent when clog_to_monitors is updated

common: reset last_log_sent when clog_to_monitors is updated

When clog_to_monitors is disabled, "last_log" still keeps increasing by
get_next_seq() if OSD writes info to clog

But "last_log_sent" doesn't increase, if we disable clog_to_monitors for
a bit longer and then re-enabling it, the num_unsent could be bigger than
log_queue_size(), it will trigger an assertion in _get_mon_log_message

We need to reset last_log_sent to last_log before updating clog_to_monitors

Signed-off-by: Gerald Yang <gerald.yang@canonical.com>