Log when an import is being skipped because we already have it

Summary:
Behavior Changes:

• Those pubkeys being imported with add_keypool set and are already in the wallet will no longer be added to the keypool

This is a partial backport of Core PR16301 : https://github.com/bitcoin/bitcoin/pull/16301/commits/fae7a5befd0b8746d84a6fde575e5b4ea46cb3c4

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6411