Started unit testing for head.go, freelist.go, store.go
Initial version of btree.

This is the first cut of disk based btree implementation that supports / will
support,
- crash-only.
- lockless.
- MVCC, copy on write.
- reusing stale blocks to avoid compaction frequency.

APIs like Insert(), Count(), Lookup(), KeySet() are breathing. But the
algorithm itself is not fully validated.