Use fast SRWLOCK locks under Windows

They are fast and also have the same semantic of Linux ones.

This allow to simplify the code and especially to use
SleepConditionVariableSRW() to wait on a condition releaseing the lock,
this has the same semantic as pthread_cond_wait().

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>