Add rate limiting to the Ethereum event oracle

[james-chf]

(this PR was split out from #253)

Closes #464

This PR has the oracle back off a bit after having gotten the latest block from the Ethereum JSON-RPC endpoint being used, otherwise we call the Ethereum RPC endpoint very fast over and over. Also adding some extra logging, so we can see what the latest block height is.

2022-08-04T12:56:01.158995Z  WARN namada_apps::node::ledger::ethereum_node::oracle::oracle_process: 
Couldn't get the latest Ethereum block height, will keep trying
error=FailedToSend(Connect(Io(Os { code: 61, kind: ConnectionRefused, message: "Connection refused" })))

2022-08-04T12:53:54.587542Z DEBUG namada_apps::node::ledger::ethereum_node::oracle::oracle_process:
Got latest Ethereum block height latest_block=Uint256(7345457)

(we could expand the backoff strategy in later PRs but adding these small 1-second sleeps solves the immediate issue)

[james-chf]

As this PR got merged into #253 , it will merge with that