iproto: refactor `iproto_req_handler` structure

So far only one type of a handler can be set for a particular request type,
but the following commit will bring two more handlers. This patch prepares
`struct iproto_req_handler` for this extension to simplify next commits.

Part of #8138

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring