Allow streaming rows from /v1/transactions requests

[jeromegn]

In SQLite, it's possible to return data as part of an INSERT or UPDATE. It could also make sense to use a transaction for reading data only (if you want to do multiple SELECT, for example).

Right now, the /v1/transactions API handler does not stream back results and in fact only accepts statements that won't return data.

It would be desirable to mimic using a plain SQLite database as much as possible via the HTTP API.

There are some challenges:

• Should it be possible to stream rows back for multi-statement transactions? Perhaps we only stream the last one or only allow 1 statement.
• Come up with a new type of NDJSON-serialize line for "rows affected"
• Make it backwards compatible (should be doable via Accept header?)