Upload packfiles directly to Sia

[cap10morgan]

This should improve performance by uploading packfiles directly to Sia Skynet rather than parsing them into individual objects and uploading those. We'll have to keep track of what's in each packfile in the chaintree (see the filesystem storage in go-git for reference). Presumably we'll want to cache them after downloading so that any other object read requests for things in the same packfile will already have what they need. That should improve cloning / pulling performance quite a bit.

[MizunaRei]

https://filebase.com/ runs a cache service for sia. It really fast. DGIT could run a cache server cloud or locally.