Fix csBestBlock/cvBlockChange waiting in rpc/mining

Summary:
Modernize best block mutex/cv/hash variable naming

This is a backport of Core PR12743

Depends on D1902

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1904