Add CWallet::BlockUntilSyncedToCurrentChain()

Summary:
This blocks until the wallet has synced up to the current height.

This is extracted from Core's PR10286 commit 5ee31726360cbe343f5a1a50a5e440db736da5b7

Depends on D2266

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2267