libsnapshot: Fix a race condition in WaitForDelete.

WaitForDelete is supposed to block until close() has been called on the
COW image. However, it could race with the destructor for Snapuserd
since nothing guaranteed it was freed within the global lock.

This patch fixes the bug and refactors the surrounding code to make the
responsibilities of each thread clearer.

Bug: N/A
Test: vts_libsnapshot_test
Change-Id: Icfc264e6dff378db585c81cde381cc24269f4800