Separate warp storage and message verification

[StephenButtolph]

Behavioral changes

1. Changed the ACP-118 error codes so not re-use error codes. If we determine that maintaining the same error codes is important.. Then we can revert this... But I highly doubt anyone uses these (and it allows better testing).
2. Allowed any warp message to be specified as an off-chain warp message. Previously there was special casing around not allowing Hash messages. It isn't clear to me why this restriction existed.
3. Removed the verification metrics. We could re-add this, but if we do we should register them through the VM registerer rather than the global.
4. Removed the signature cache. We should probably re-add this, but we don't need to actually have a reference to the cache. I feel like it would make the most sense for us to just always cache the signatures in the acp118.Handler, and then remove the NewCachedHandler function.

Cleanup

1. Removed message parsing from the Backend type, it is handled by the Config now
2. Removed message signing from the Backend type, it is handled by the acp.118.Handler
3. Split Backend into 2 types, Storage and Verifier.
4. Simplified (imo) the Verifier logic
5. Simplified the tests of the Storage and Verifier.
6. Passed around concrete types rather than interfaces.
7. Moved files in the warp/backend package into the warp package.

Other

I also just removed the log from Backend.AddMessage, we could provide the logger to the Storage... But I felt like it wasn't worth it... Happy to re-add it if we have found this log useful in production.

[StephenButtolph]

Sorry... The diff is super annoying to review because of the renames:

• warp/backend/backend.go -> warp/storage.go
• warp/backend/backend_test.go -> warp/storage_test.go
• warp/backend/verifier_backend.go -> warp/verifier.go
• warp/backend/verifier_backend_test.go -> warp/verifier_test.go

[ceyonur]

left a small comment regarding to preserve the caching, otherwise lgtm.