Improve memory usage in federated metastore request/response forwards

[ddcprg]

Spike

Overview
At the moment the database mappings create copies of the Thrift objects it processes. Some of these objects can be huge and the creation of copy will demand memory for the request. As we only need the original object to determine which megastore will process the request we could mutate the current object instead of making a copy.

Acceptance criteria
Evaluate memory usage before and after the change.