Rename sigOps to sigChecks in txmempool

Summary:
We don't use sigops anymore, so update the variable names and comments accordingly.

There is no change in behavior.

Test Plan:

ninja all check-all
ninja bitcoin-fuzzed # make sure it's not broken

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12783