Replace synchronization in XARM with CHM.

- Doubles read query throughput for concurrent MATCH (n) WHERE id(n)=x RETURN n.name
- Moves write contention for small writes solely into LogBuffer.flush() and
  PersistenceRow flushing.
Removed synchronization in createTranasction path

This removes the last coordination point on small tx hot path,
there is now no visible coordination between threads running small
read queries.

- NeoStore now uses AtomicLong for lastCommittedTx
- XIDImpl now uses AtomicLong for sequence id generation
Merge pull request #2195 from jakewins/2.1-readconcperf

Replace synchronization in XARM with CHM.