[rpc] Finish backporting changes to prioritisetransaction's priority_delta argument
Summary:
prioritisetransaction RPC had this argument removed in PR9602, then re-added in
PR10252, then re-named in PR10488.
https://github.com/bitcoin/bitcoin/pull/9602/commits/f9b9371c6027905f73a2558d6bcaca8a355c28a6
https://github.com/bitcoin/bitcoin/pull/10252/files
https://github.com/bitcoin/bitcoin/pull/10488/files
Since the intermediate states are undesirable and I already did the renaming
to 'dummy' in D4812 (landed), this pulls together the overall remaining effects
of the PRs which are ultimately small:
- modify documentation slightly and force argument to be 0 instead of ignoring.
- change RPC tests to not pass priority, and use keyword arguments instead.
- simplify mining_prioritisetransaction test slightly (from 9602).
The release note is taken verbatim from Core's 0.15 release notes.
Test Plan: ninja check-all
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D4829