This adds a templated StatementCache helper object (with tests!) that allows
consumers of storage to easily cache and reuse statements. Consumers only need
to pass in the query string, and the rest is handled for them.
r=asuth
sr=rs
This gets rid of the old, unused native code test file for storage. It adds two
more test files which test our C++ helper classes to ensure that they behave as
advertised. Also fixes a bug found in mozStorageTransaction that was uncovered
while writing these tests.
r=asuth