Windows: Improve locking in threading abstraction

Windows: Improve locking in threading abstraction

Convert the usbi_mutex_t type to a CRITICAL_SECTION object. There
are numerous advantages including lower resource usage and a better
fast-path (doesn't require entering kernel space).

Simplify the condition variable implementation by not associating a
wait structure with a particular thread ID. This is not needed and
causes an unnecessary search through the linked list of any available
wait structures when the real optimization is just reusing an already
created event object.

Also, while here remove all the checks for NULL pointers because we
don't do such a silly thing inside the library.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>