[services] Tunnelbroker - Change findMessageItem, removeMessageItem to use composite key

Summary:
As we changed the messages table model to use a composite key in ENG-1362 we can use a composite key instead of an index in findMessageItem, removeMessageItem methods to improve database lookup performance.

Linear task: ENG-1493

Test Plan: Run database-related unit tests using yarn run-unit-tests

Reviewers: karol, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek, adrian, atul, karol, abosh

Differential Revision: https://phab.comm.dev/D4694