Handle multi-command queries correctly inside SQL as well as plpgsql functions.

Handle multi-command queries correctly inside SQL as well as plpgsql functions.

Postgres-XL sends down utility statements to the remote nodes as plain query
strings. When there are multiple commands in a query string, separated by ';',
we were incorrectly sending down the entire query string again and again while
handling each command. This can lead to unpleasant as well as incorrect
behaviour. This was earlier handled for execution via psql, but this patch
fixes it for SPI and other places such as extension creation and SQL function
handling.