Add cached blockhash that updates based on a fixed expiration interval

[jarry-xiao]

Right now, we fetch the blockhash every time we send a transaction, but in theory this makes a few unnecessary RPC calls. One way to potentially solve this problem is to spin up a thread that polls on an interval up update an Arc<RwLock<Hash>> on the client object.

Reduces RPC overload and latency.

Low-ish priority

[jarry-xiao]

Maybe instead of polling thread, keeping track of the last fetch time is a cleaner solution. Still need the Arc<RwLock<Hash>> for concurrency, but this makes the fetch on demand