Factor out CWallet::nTimeSmart computation into a method.

Summary:
No change in behavior, this change just pulls some code out of
CWallet::AddToWallet that was making it very long into a separate method.

Backport of core's PR9369

Test Plan:

make check

Reviewers: schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D943