New WebLocks implementation impacts existing VFS classes

[rhashimoto]

The WebLocks implementation has been changed to fix bug #51. This bug had no significant impact for applications, but the fix does change the example IndexedDB and Origin Private File system VFS classes. The change provides each open file with its own locking manager, rather than sharing a manager on the VFS class.

I wasn't planning on fixing this bug but it turned out that doing so made writing unit tests a little nicer. I wrote true unit tests for most of the example VFS classes (as opposed to exercising them only through the SQLite API), and this fix came along as a side effect.

The changes to the example VFS classes are pretty straightforward so if you have your own customized classes they can be upgraded mostly by copying these diffs for IDBMinimalVFS.