Lots of TreeService cleanup

After realizing the default collation mode was only set from TreeServiceImpl, and hence still LOOSE when on FDB, got sucked into more cleanup.

Nothing particularly stunning:
- Remove treespaces. Fragile and never used.
    - Fun fact: two of the targeted tests *didn't even care* when it was removed.
- Collapse TreeLink and TreeCache.
- Shuffle a couple test helper methods so no general test paths is attempting to get a TreeService
    - Allows getting rid of NopTreeService. TreeService is now used *only* when Persistit is active.
- Import cleanup, dead code deletion, etc