Abstract out SignatureChecker
Make signature cache optional
Replace SCRIPT_VERIFY_NOCACHE by flag directly to checker
Merge pull request #4890

e790c37 Replace SCRIPT_VERIFY_NOCACHE by flag directly to checker (Pieter Wuille)
5c1e798 Make signature cache optional (Pieter Wuille)
c7829ea Abstract out SignatureChecker (Pieter Wuille)