Overhauled Dsync with new API, P2P support, Push & Pull hooks

We've completely reworked dsync to make the API easier to work with, and add support for p2p as a dsync transport!

This new API includes configurable hooks integrated into the lifecycle of push & pulls. Check out ExampleNew in dag/dsync/dsync_test.go for an annotated overview.

We're using this new dsync work as the basis for "remotes" in qri, which makes for a nice production-grade example.