Implement batched writing

ATM, writes to a remote server must be staged locally and then all new data gets sent up on commit. This is wasteful in at least two directions and, in particular, slows down sync.

Needs clear design.

Blocked by:

• https://github.com/attic-labs/noms/issues/2640
• https://github.com/attic-labs/noms/issues/3178