[wallet] Move getBlockHeight from Chain::Lock interface to simple Chain

Summary:
Add HaveChain to assert chain access for wallet-tool in LoadToWallet.

Backport of Core PR16426 part [2/5] : https://github.com/bitcoin/bitcoin/pull/16426/commits/de13363a472ea30dff2f8f55c6ae572281115380

Depends on D7934

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7935