Implement GetKeypoolOldestTime and only display it if greater than 0

Summary: Backport of Core PR16528 [26/43] :https://github.com/bitcoin/bitcoin/pull/16528/commits/f1ca5feb4ad668a3e1ae543d0addd5f483f1a88f

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8445