Merge #10196: Bugfix: PrioritiseTransaction updates the mempool tx counter

Summary:
6c2e25c [qa] Test prioritise_transaction / getblocktemplate interaction (Suhas Daftuar)
acc2e4b Bugfix: PrioritiseTransaction updates the mempool tx counter (Suhas Daftuar)

Tree-SHA512: dcf834df52d84d5eb86acb847c3f28d3cffd1f78f3092b8ff8913c2c400675a071c48a19cd852fdbaac1582aa1dba23433e0e16055831ef2a5e76dde91199941

Backport of Core PR10196
https://github.com/bitcoin/bitcoin/pull/10196/

Test Plan:

make check
test_runner.py

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

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3386