[mining] Rename several CBlockTemplateEntry members for clarity

Summary: Rename variables in order to clarify what these items are associated with for a future change, which will be adding package data. Additionally, add the txSize which will be used in further diffs.

Test Plan:

make check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2862