Various nits in the signature cache machinery
Various nits in the signature cache machinery

Summary: As per title.

Test Plan:
  make check

Reviewers: freetrader, CCulianu, #bitcoin_abc, dagurval, sickpig

Reviewed By: #bitcoin_abc, dagurval, sickpig

Differential Revision: https://reviews.bitcoinabc.org/D524

Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,cculianu/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,cculianu/bitcoin-abc,cculianu/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,cculianu/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,cculianu/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,cculianu/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,cculianu/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,cculianu/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc