Move ConnectionIdentifier to util package.
Replaced use of MongoIdentifier with ConnectionIdentifier
Updated mapper.ConnectionIdentifier to util.ConnectionIdentifier
Issue #1505 - Create RecordRule for field name.
Merge pull request #1556 from lift/tcn_issue_1555

Move ConnectionIdentifier to util package.

I also replaced the use of MongoIdentifier with ConnectionIdentifier.

Then, I added RecordRules.fieldName that takes a ConnectionIdentifier as an
argument. I had to add def connectionIdentifier to MetaRecord for this to work.