allow mongodb connection handles to include an authorization database specification

[sierra-moxon]

currently supported handle for mongodb connection parameters:

mongodb://localhost:27017/test_db

also for mongo, if we want to specify authentication params in our handles, we had some extra characters at the end of the handle like this:

mongodb://test_user:test_password@localhost:27017/test_db?authSource=admin

we need to adjust the _db_name function that extracts the db name from the handle to take authSource param into account.