(#1068) - cache websql dbs

Safari 7 starts throwing DOM Exception 11 if
we open up too many db handles, which crashes
our unit tests. Caching them fixes that.

We actually already tried this once before in
6575e8d490fbc745242edf72090598a72df6f120, but
since db.destroy() didn't use the cached
versions, the tests still failed.