Windows: Hash table improvements

Windows: Hash table improvements

1) The current implementation was not entirely thread-safe. The hash
   table was read outside of the mutex, which in some cases could lead
   to improper hashing.
2) The hash creation function accepted a size parameter that was
   hard-coded. Eliminating this saves time because we can ensure that
   the pre-defined size is a prime number instead of requiring code to
   calculate a prime number.

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