Token client improvements

[CriesofCarrots]

It's not that easy to use spl-token-client with an RpcClient (to point at a test-validator or live cluster RPC).

Changes:

• (First commit is purely cosmetic, and can be ignored in review)
• Use the nonblocking RpcClient instead of the blocking one (we're already async here)
• Arc-wrap the RpcClient in ProgramRpcClient so that it's easier to use for multiple calls without will not live long enough compile errors
• Change RpcClient send impl method from send_transaction to send_and_confirm_transaction - I think this is the most controversial of the changes. This approach seems the most straightforward to me, but we could instead require the caller to do any confirming they need/want. If so, we probably need to rework some things in client/src/token.rs. For instance, Token::create_mint() doesn't return the Output (signature) from process_ixs(), so there isn't currently a way for a caller to confirm that signature.

[joncinque]

Looks great! Changing to send_and_confirm_transaction is more aligned with process_transaction on the banks client anyway. If there's a need to send transactions without confirmation, we can implement a new trait for that with Output = Signature, right? This client is really forward-looking!

[CriesofCarrots]

Here's a little example of using this client against a solana-test-validator, btw: https://gist.github.com/CriesofCarrots/0539fec5c5601db36d5b7a0c0ead385a

[CriesofCarrots]

CI failure seems due to changes upstream in web3.js/transaction.ts (twotx patch), but I'll fix that before merging this

Pull request has been modified.