Add calls to CWallet::BlockUntilSyncedToCurrentChain() in RPCs

Summary:
This prevents the wallet-RPCs-return-stale-info issue from being
re-introduced when new-block callbacks no longer happen in the
block-connection cs_main lock

This is extracted from Core's PR10286 commit 5d67a7868db188f7e43ce9028f215034d057790d

Depends on D2267

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2268