Safety check for agreement of unexpected msg size limit between peers

[carns]

If the client and server disagree on the maximum unexpected message size (for example, if each is choosing them at runtime based on interface settings) then it may cause communication problems. It would be nice if this scenario produced an error msg or negotiated to the smaller of the two sizes.

Maybe the unexpected message size should be in the header of the Mercury msg so that the receiver could at least throw an error?

This is a low priority ticket, it isn't blocking anything. I'm mainly just recording here for future reference.