Fast-forward syncing, rebased.

[ncalexan]

Here's a sliced-and-diced version of #563, with the simple stuff pulled forward.

I'll dig into the larger restructure for atomic metadata updates now, but perhaps you could stamp the easy bits @grigoryk?

[grigoryk]

I've updated the base PR for this (#563) with the split introduced here and a patch to:

• correctly skip over datoms while skipping a transaction
• skip over the second transaction, assumed to be user schema definition (since we can't merge yet)

[grigoryk]

Work continues in #563 .