Replace the mocked store on top of TestTsdbStore with a MemoryStore

This replaces most mocking that was done of `TestTsdbStore` with an actual `TsdbStore` implementation that was mostly copied from what `MockBase` was using. The interesting changes are in `test/storage/MemoryStore.java` and `test/storage/MockBase.java`. Besides those all tests have just been updated to use `MemoryStore` instead of `MockBase`