iproto: introduce is_in_replication connection flag

Connections that are used for replication handled a bit differently. For
example during processing JOIN or SUBCRIBE requests all IO is done
outside of iproto. Thus we need to drop regular and replication
connections differently. Let's introduce `is_in_replication` flag for
this purpuse. While at it we can refactor other pieces of code where we
differentiate connections to use this flag.

Part of #8423

NO_TEST=refactoring
NO_CHANGELOG=refactoring
NO_DOC=refactoring