Allow asynchronous codecs

[christianbundy]

This resolves #13 by wrapping codecs in Promise.resolve or Promise.all for batch operations, and adds an async test which is just the flumelog.js test with setTimeout. This shouldn't have any effect on synchronous codecs.

I don't think we're currently handling codec errors, so I've left reject() unused, but if there's an elegant way to handle promise rejections I'd be happy to implement it.

[dominictarr]

reflecting on this a while... async codecs don't make sense: a codec is just a decoding format. that we are doing decrypting inside the codec in secure-scuttlebutt is an ugly hack that only works because decrypting is sync. I don't want to expand that hack by making codecs officially async.

However, we could add an async map step in flumedb... it would wrap the log, affecting the output of log.stream and log.get via an async function... then it would work with any sort of flumelog, not just flumelog-offset (note: I am working on http://github.com/dominictarr/flumelog-random-access-file which has a different format and is much faster than flumelog-offset)

[christianbundy]

Thanks! I'm not familiar with flumedb, but I'll look around and see if I can get another prototype working. Thanks again for taking a look at this!