Suggested https://github.com/bitcoin/bitcoin/pull/17304#discussion_r341194391
by Gregory Sanders <gsanders87@gmail.com>
Reason for keeping the return true return false verbosity is that more code
will be added after the ReserveKeyFromKeyPool() call before returning.
https://github.com/bitcoin/bitcoin/pull/17381/commits/bfd826a675445801adec86a469040f3ceb8172ee
Depends on D7418
Partial backport of Core PR17381