Use camelCase for function names

Renames percentile_cont and percentile_disc to percentileCont and percentileDisc respectively
Merge pull request #1189 from cleishm/camelcase_funcs

Use camelCase for function names
Make merge use uniqueness constraints and correctly take locks

o Acceptance tests for Merge using constraints
o Introduce SchemaIndexKind to SchemaIndex start points
o Extend PlanContext to support lookup of unique indexes
o Add support for testing failing queries to DocTestBase
o Introduce ReferenceNodeAcceptanceTest
o Introduce DeleteReferenceNode helper for GraphDatabaseTestBase