[WIP] write path prototype

[zachschuermann]

Write path prototype for supporting blind appends. Major pieces:

• Transaction: an in-progress write to a table. follows a builder-type pattern to add writes and in the future stage deletes, evolve schema/metadata, etc.
• WriteContext: everything executor needs to write data
• an API layer for communicating 'write metadata': every executor returns data about the write which is passed to Txn.add_write_metadata(...) to stage the writes for commit. (naming TODO)
• Engine API: JsonHandler::put_json

Open questions/TODOs:

• can we stream commits? so that the write_metadata passed back to the transaction is effectively streamed into the commit in order to prevent forcing the engine to materialize all actions into memory at once
• main thread -> executor boundary: do we want to communicate write_context as EngineData?
• need to move a bunch of engine code from the write test into default engine.