iproto: clean accounting of processed messages in input buffers

Message body resides in one of rotating input buffers for the
connection. When we don't need message body anymore we advance the
reading end of the input buffer by the size of the message. But message
processing order can differ from the order of messages in the wire.
Thus this advancing a bit hacky. Let's instead mark the data in the
input buffer as read when we process all the messages in the input
buffer. We cannot reuse the buffer any earlier anyway.

Follow-up #7327

NO_TEST=refactoring
NO_CHANGELOG=refactoring
NO_DOC=refactoring

(cherry picked from commit 9e918278)