Improve Sync

[ghost]

Noms sync has been a work horse for us, but it's time to address it's current deficits:

• Syncing datasets with very long commit chains can take a very long time. (nomfs, for example)
• Syncing degrades chunk locality within nbs. We need to sync to optimize locality.
• Sync requires staging all data in a temporary store locally before writing into destination.
• Reading with ReadManyValues would dramatically improve perf of syncing from nbs/aws.
• Sync needs a way to sync an arbitrary value to a dataset (as opposed to requiring the source to be a commit).
• Struct type validation dramatically reduced sync speed. Remove it.