WL#1617: merge run_lock, start_cond and stop_cond of the slave I/O and SQL thread into one

No need to have mi->run_lock and mi->rli.run_lock, we can have one mutex instead
of these 2 (cases where one is acquired and not the other are quite rare, so we
would benefit by having only one, as far as speed and code simplicity is
concerned). So simply drop mi->rli.run_lock and use mi->run_lock instead, after
checking that it introduces no bug and updating comments (slave.h).
The same way, drop mi->rli.start_cond and mi->rli.stop_cond (check that
mi->start_cond mi->stop_cond and those 2 are always used inside a while(), then
it's safe to do the replacement).
To do in 5.0, as it's:
- not a small code change (difficult merge maybe)
- relatively safe change