Introducing GraphDatabaseService.execute(...)

Enables executing cypher queries without having to keep track of an
ExecutionEngine instance, instead the database keeps the ExecutionEngine
internally.
Unignored tests in Cypher.

Mostly just unignoring tests, but some small fixes to get green.
Merge pull request #3410 from thobe/2.2-gds-execute

Introducing GraphDatabaseService.execute(...)
Merge pull request #3603 from pontusmelke/unignoring-tests

Unignored tests in Cypher.
Serialize identifiers in JSON representation of the Cypher execution plan description