If a replica is actively delaying a transaction when restarted (STOP
SLAVE/START SLAVE), when the sql thread is back up,
Seconds_Behind_Master will present as 0 until the configured
MASTER_DELAY has passed. That is, before the restart,
last_master_timestamp is updated to the timestamp of the delayed
event. Then after the restart, the negation of sql_thread_caught_up
is skipped because the timestamp of the event has already been used
for the last_master_timestamp, and their update is grouped together
in the same conditional block.

This patch fixes this by separating the negation of
sql_thread_caught_up out of the timestamp-dependent block, so it is
called any time an idle parallel slave queues an event to a worker.

Note that sql_thread_caught_up is still left in the check for internal
events, as SBM should remain idle in such case to not "magically" begin
incrementing.

Reviewed By:
============
Andrei Elkin <andrei.elkin@mariadb.com>