API for transfer progress stats

[whyrusleeping]

There has been a fair amount of demand for a way to check the 'transfer progress' of a given file, similarly to how torrent clients show.

This is not currently a very easy thing to track, but i think that we can abuse context values and the commands diagnostics api (ipfs diag cmds) codebase to get something working here.

We first should design what we want this api to look like, and what format the information it provides should be in. Then we can go ahead and try to implement it.

[adamlawrencium]

@whyrusleeping Is this still of interest?

[whyrusleeping]

@adamalawrence Yeah, it is.

We've made a small bit of progress on this since i posted this. In the ipfs pin add code, we put a 'progress tracker' in a context and pass it down through the calls to help us get a small amount of progress information.

see: https://github.com/ipfs/go-ipfs/blob/master/core/commands/pin.go#L84
and: https://github.com/ipfs/go-ipfs/blob/master/merkledag/merkledag.go#L167

for how its being used.

We definitely want more fine grained information about transfers than that (and we need to improve the UX of it a bunch), so theres still things to be done for this issue