Make IPFS add and cat calls more asynchronous through threading

[machawk1]

Both replay and indexer perform IPFS.cat() and IPFS.add() for each of the header and the payload in series. A potential optimization exists in threading these two calls with a timeout instead of using a timer (SIGALARM in the case of the replay). This has the potential to have big ramifications, as the process is executed for each embedded resource as well on replay.

/cc @ibnesayeed

[machawk1]

Some of the promise-esque constructs are only available in fairly modern version of Py3.

[DonaldTsang]

@machawk1 Maybe this can be helped too #388

[ibnesayeed]

@DonaldTsang: Maybe this can help too #388

I can't see how this ticket is related to the referenced request.

[DonaldTsang]

Sorry, that was a typo, I was meant to say that Async can be combined used for #388

[machawk1]

A relevant repo for benchmarking IPFS (js implementation cf. go implementation) is also in-the-works.