[backport#17999 1/2] Add ChainClient setMockTime, getWallets methods
Needed to set mock times, and get wallet interface pointers correctly when
wallet code is running in a different process from node code.
Partial backport of Core PR17999
ninja check check-functional
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D6995