[backport#15632] [wallet] Keep track of the best block time in the wallet

Summary:
Move nTimeBestReceived (which is only used for wallet
rebroadcasts) into the wallet.

https://github.com/bitcoin/bitcoin/pull/15632/commits/f463cd107361a172a17e4c5510b06eb8a67aade0

Depends on D5947

This is a partial backport of Core PR15632

Test Plan:

cmake .. -GNinja
ninja check check-functional

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5948