Fix test database clearing logic.

Clear test database after every test in DataTransfer tests, and after
entire fixture runs for FastQuery acceptance tests so that tests run quickly.
Implement partial fast query functionality.

Fast query for explicit paths, descendants, children implemented.
Simple predicates of form [@@xxxx = 'value'] implemented.
Not done:
-> Efficient implementation of //*[predicate]
-> Predicates with "and/or" or inequality.
-> Field value lookup.