iproto: factor out sql request processing to box_process_sql

We are planning to add access checks for EXECUTE and PREPARE requests.
(Currently, everyone, even guest, may execute these requests.)
Checking access in tx_process_sql(), which is defined in IPROTO code,
would violate encapsulation and look inconsistent with other request
handlers. Let's move the code that actually processes an SQL request
to the new function box_process_sql() taking sql_request and returning
the result in a port object.

To unify handling of all SQL requests in box_process_sql(), we add a new
format for port_sql - UNPREPARE. The format works only for dumping port
content to MsgPack buffer - it encodes an empty map then. This way, we
don't need to return the is_unprepare flag from box_process_sql().

Needed for #8803

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring