Multiple batched request

[Ezeta31]

it would be useful to be able to submit multiple requests in one batch, and to retrieve the multiple results.

The idea is to avoid several way and back from the swift app to the server when we do not need to wait for the server answer to decide the next request (ie. Request several select on various table without join, and retrieve the different select result in one answer)

[gwynne]

This functionality will be available via the CLIENT_MULTI_STATEMENTS and CLIENT_MULTI_RESULT capabilities implemented in the in-progress package rewrite. You can follow this branch to see progress (at least until I manage to find time to make an actual PR for it).