[backport#17999 1/2] Add ChainClient setMockTime, getWallets methods

Summary:
Needed to set mock times, and get wallet interface pointers correctly when
wallet code is running in a different process from node code.

https://github.com/bitcoin/bitcoin/pull/17999/commits/1dde238f2c21a0cc9bada10a2449cf9c6b2178ad

Partial backport of Core PR17999

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6995