remove mtype/rtype htex interchange<->worker dual type fields #3950
Description
Is your feature request related to a problem? Please describe.
Prior to PR #3871 there were two channels of messages from workers to the interchange. PR #3871 merged those into one channel. That PR kept two different message tagging systems used for messages on those two channels - the mtype and the rtype.
Describe the solution you'd like
I don't think those two tag systems need to exist, and that the wire protocol can be simplified into using a single tag type/dispatcher.
Additional context
tag @khk-globus because he's actively poking at this, and @yadudoc who has expressed interested in protocol maintenance work previously.