Store a tokio::runtime::Handle in alloy/ethers DB

[DaniPopes]

Store an instance of tokio::runtime::Handle by calling Handle::try_current in the constructor instead of calling it on every RPC call.

This also removes the need to create a tokio runtime for every RPC call as well by always requiring a tokio handle to be present at instantiation.

The check for current-thread runtime should also be removed, because we don't want to spawn a runtime as above.

This change was also made in Foundry, see foundry-rs/foundry#7860