Add methods to find subsets on composite sets
Add methods to find subsets on composite sets 2
Stores: add assertion on negative recid
Trees: more negative recid assertions
StoreDirect: even more assertions for negative recids
StoreWAL: fix some TOMBSTONE details
Maven: run unit tests in parallel
#167: Add a makeOrGet to DB Collection maker API.
Fix #167: Add DB.exists() method to check if named record/collection exists.
Fix #167: Fixed mistake in previous commits