Merge #9533: Allow non-power-of-2 signature cache sizes

Summary:
7482781 Allow non-power-of-2 signature cache sizes (Pieter Wuille)

Tree-SHA512: 5731c22b46c5ae81cf6d52000c28e39b243a47d96d91079942a5b5a10db214449217f71aa2195e18f8a3917cb206b04c75dc13e4522eb700a1dbf1819013ba22

Backport of Core PR9533
https://github.com/bitcoin/bitcoin/pull/9533/files

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3175