Fix sol msg verifier logic (and add cache)

[eloylp]

This PR closes #7. It accomplishes the following:

1. Adds a small cache for the Sol RPC client to handle scenarios where multiple Axelar messages are in the same Sol transaction.
2. Utilizes the RPC client with the cache in the message verifier.
3. Simplifies the message processing logic in point 2, enabling iteration per each message to decide the vote result on a per-message basis.
4. Previously, the Vec<Vote> was being overwritten, resulting in incorrect votation. It would be beneficial to allocate additional time to implement unit tests for this functionality. Here we are only fixing the production code.

Review Tip: Please review each commit separately.