[sync] Remote transaction validation

[grigoryk]

Tolstoy needs to be able to sanity-check incoming remote transactions. This will likely start off as a stub validator, and gradually evolve.

Super basic: parent matches, IDs in ranges, types match schema…

More advanced: include schema changes inside incoming txes, internally consistent remote txes, …

[rnewman]

Remember that transactions can themselves alter the schema, so validation is, in part, a speculative write. Fortunately we have SQLite transactions locally, so we can roll back!

A much larger question is: what do we do if an error is found?

[grigoryk]

Completed in #563 - there's some basic "does this make sense" logic, but largely transactor is used as a "validator" (speculative write). On errors we gracefully abort - but at that point, there's currently no "recovery flow" that will get clients syncing again - unless remote state changes.