winpr: improve and fix locking for data structures

- Improved/completed(almost) winpr's critical section implementation
- Replaced WaitForSingleObject locking with critical sections

Note:
WaitForSingleObject should _never_ be used for granular low-contention
locks as it _always_ enters the kernel.

Just replacing WaitForSingleObject locking in Bufferpool with
EnterCriticalSection boosts the multithreaded rfx decoder
performance by almost 400% on win32.