Let BlockAssembler::AddToBlock take a mempool entry

Summary:
This makes it easier to reason about and will simplify the follow up diffs.
There is no change in behavior.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13395