[Query engine] Support multiple statements per simple query

[levkk]

Description

Support the client sending multiple queries inside a Query message (simple protocol), e.g.:

Query("BEGIN; SELECT 1; COMMIT")

We would need to rewrite these queries into multiple ClientRequest and push them through the query engine serially.

More info

• Improve JDBC Postgres driver support #375
• We don't need to do this for extended protocol because Postgres doesn't allow multiple queries inside a single prepared statement anyway.