Support for async_insert for prepared blocks sent over the native protocol.

[alexey-milovidov]

Use case

Many clients are concurrently sending data using Distributed tables.

Describe the solution you'd like

Deserialize blocks as usual, put them in the queue for async inserts, and squash them.
This is slightly different from how async inserts operate, but in fact, even simpler (no parsing).

[CurtizJ]

Do we want to support INSERT SELECT?

[alexey-milovidov]

Do we want to support INSERT SELECT?

It's unnecessary - if it has any additional complexity, it is to skip.