Track keypool entries as internal vs external in memory

Summary:
This resolves a super minor performance regressions in several
keypool-handling functions

Backport core's PR10235

Test Plan:

make check
./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1043