Adds trace logging statements during election and cluster join/leave.

This is a temporary commit, because we need the execution powers of the CI
 system which seems capable of reproducing some edge case failures. After this
 task is done it should be reverted and if prudent, polished and reapplied.
Merge pull request #1420 from boggle/master

Update Cypher test dependencies
Add GraphDatabaseService.mergeByConstraint (Core API version of merge)

o Provides a utility for adding unique entries for a constraint (getOrCreate) with proper locking, i.e.
  transactions using mergeByConstraint block each other
o Add Merger interface and MergeResult class to core API
o Add documentation
o Add UniqueFactory.asMerger to integrate existing API (UniqueFactory may be deprecated in the long term)
Update documentation examples for creating unique nodes