Create a typedef for handling mapDeltas in the mempool.

Summary:
This commit creates a typedef for handling mapDeltas in the mempool.
This exists to allow easy changes to the base type going forward.

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1383