Remove uses of chainActive and mapBlockIndex in wallet code

Summary:

This change removes uses of chainActive and mapBlockIndex globals in
wallet code. It is a refactoring change which does not affect external
behavior.

Backport of core PR14711.

Test Plan:

ninja all check-extended

Ran the tests with UBSAN and ASAN as well.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5683