Adds constraints warning to REST docs as well.
Re-introduces filtering during recovery, which was disabled when the code path for recovery updates and
normal updates was split into two separate ones. Added a unit test to avoid regression.

This fixes #1092
Unused commits.
Updated id space text of property keys and label names
Validate uniqueness constraints for new data.

Support concurrent updates of data affected by the constraints.
Acquire locks to ensure mutual exclusion.
fixed license header
Introduce UnboundValue for keeping track of missing optional nodes.

o ClosingIterator.materialize converts UnboundValue to null
o Equality behaves correctly when comparing against UnboundValue
Fixed typo in linux-installer README

It is 'java-package', not 'java-packaging'
Merge pull request #1096 from apcj/2.0-optionally-exists

Introduce UnboundValue for keeping track of missing optional nodes.