Caching SCW Verifier for server-side use

[neekolas]

Follow up to #1070

One thing that would be really nice for the server is a SmartContractSignatureVerifier that caches the results for a given input in memory.

If you imagine a user getting added to a 200 person group, all 200 members are going to have to go and verify that same signature. Will be both faster and cheaper if we can cache the result.

Since the results are immutable we can theoretically cache forever, but we should give it a bounded size so we can never run out of memory getting spammed. Maybe a LRU cache.