Todo

the ethermine rpc is usually fastest. but its in the private tier. since we only allow synced rpcs, we are going to not have an rpc a lot of the time
- if not backends. return a 502 instead of delaying?
tarpit ratelimiting at the start, but reject if incoming requests is super high
thundering herd problem if we only allow a lag of 0 blocks. i don't see any solution besides allowing a one or two block lag
add the backend server to the header?
the web3proxyapp object gets cloned for every call. why do we need any arcs inside that? shouldn't they be able to connect to the app's?