Introduce nodeGetUniqueFromIndexLookup

o Introduce StatementConstants.NO_SUCH_{NODE, LABEL, PROPERTY_KEY}
o Introduce uniqueIndexGetForLabelAndProperty
o Take an index lock for nodeGetUniqueFromIndex
o Exposes nodeGetUniqueFromIndex from Cypher as exactUniqueIndexSearch
o Rename SafeProperty to DefinedProperty
Merge pull request #1167 from boggle/2.0-merge-locking

Introduce nodeGetUniqueFromIndexLookup