Add Idempotency to ILP over HTTP

[gakonst]

We can probably add idempotent retries to the client/server of ILP over HTTP, similarly to the one in the engines.

I think this is slightly different as we cannot assume that the peer will eventually reply.

Do we retry Prepares? Fulfils? Both?

[emschwartz]

This would be a protocol change so it might be worth proposing on the RFCs repo as well.

It may be possible to add this in a backwards-compatible way using headers to do the version negotiation.

Adding retries for Fulfills/Rejects is definitely more valuable, but it might be worth adding it for Prepares as well while we're at it. The benefit of being able to retry Prepares is that if the connection drops for some reason while you're waiting for the response, you could more quickly retry the request instead of waiting until the timeout to see if the peer sends you the response (indicating they got the original request).

[kincaidoneil]

Opened a strawman PR to add this to the spec here, feedback welcome: interledger/rfcs#567