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

Summary:
Instead of calling getHeight, we rely on CWallet::m_last_block
processed_height where it's possible.

Backport of Core PR16426 part [1/5] : https://github.com/bitcoin/bitcoin/pull/16426/commits/b855592d835bf4b3fb1263b88d4f96669a1722b1

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7934