Fix CTxMemPool::ApplyDeltas to accept a reference

Summary:
Changes CTxMemPool::ApplyDeltas to properly accept a reference
for nFeeDelta. This parameter was intended to be a returned
value. It was mistakenly changed as a larger refactor.

Fixes T109

Test Plan: make VERBOSE=1 check && qa/pull-tester/rpc-tests.py

Reviewers: deadalnix, freetrader, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Maniphest Tasks: T109

Differential Revision: https://reviews.bitcoinabc.org/D618