Allow syncs to recover midway, on unreliable, dropping connections

[humphd]

We should allow a file sync to pause and continue, without having to be restarted from zero. This would mean caching the initial bit until the rest arrives.

@thisandagain says "Charles Proxy" is one way to test this.

[alicoding]

I'm not sure about Firefox at this point, but I know that Chrome recently include this functionality in their Developer tools, so just another option here :dancer:

[humphd]

We're going to move this to future, and not try to tackle it until we understand per-file sync characteristics through observation.