ADD mongoUpdate metadata storage for new entities at mongoBackend
ADD stubs for other mongoUpdate medadata storage cases
ADD actionType UPDATE mongoBackend custom metadata (all test cases working, except updateMdNoActualChanges)
ADD all the UPDATE and APPEND cases with custom metadata working at mongoBackend
ADD new queryContext with custom metadata at mongoBackend
FIX testharness previously broken by custom metadata stuff implementation
ADD custom_metadata_update_query test harness and some fixes in the code to make it work
ADD custom_metadata_onchange harness test
ADD entry in CHANGES_NEXT_RELEASE files
ADD custom_metadata_ontimeinterval harness test
FIX remove LM_M()
FIX typos in comments
FIX issues raised in code review
Merge pull request #347 from telefonicaid/feature/252

Custom metadata