Addresses #1730 Put in attempted fix for race condition.

Closign and opening a cachefile is now threadsafe (race condition when trying to open during a close).
If race condition hits, it will stall the open till the close is done.
All opens are now single threaded.  This can be improved if it causes a bottleneck.

Added comments for possible future race conditions after lock refinement.

git-svn-id: file:///svn/toku/tokudb@11644 c7de825b-a66e-492c-adef-691d508d4ae1