shared: Merge BOpenHashTable in; remove OpenTracker's OpenHashTable.

The HashMap and HashSet classes are copied from userlandfs. The
HashMap one works as-is as it's already used in userlandfs; the
HashSet does not even compile yet.

Change-Id: I1deabb54deb3f289e266794ce618948b60be58c0
Reviewed-on: https://review.haiku-os.org/c/1041
Reviewed-by: waddlesplash <waddlesplash@gmail.com>