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.
I think this argument should be called `cacheWrite` or `cacheStore`, to make it apparent that the cache is always used, but the difference that this  boolean makes is whether the result is stored.