Make signature caching optional, and move it out.

This will make it easier to export the script verification code to a thread-agnostic library.

By moving some of the parameters into a SignatureChecker object, some of the long argument lists are reduced.

Builds on top of (a rebased version of) #4555.
Indeed, the `false` should be 0 or SCRIPT_VERIFY_NONE, but I didn't want to touch that here.