Separate HaveKey function that checks whether a key is in a keystore
Summary:
Functions for later use. See D4202.
Partial (1/4) backport of Core PR12560
https://github.com/bitcoin/bitcoin/pull/12560/commits/dd3c07acce7ab67f32a79393abf6227009bd440d
Test Plan:
make check
Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc
Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D4201