WIP: Eager tag negoatiation

[inducer]

Headed towards fixing #223, but that's pointless right now. Putting this here in case we need it at some point.

It's pointless for now for the reasons outlined this comment:

grudge/grudge/trace_pair.py

Lines 388 to 391 in 9e62502

	# FIXME This asserts that the whole tag negotiation process
	# is pointless. Unless there is a way to have eager communication
	# for more than one tag pending at the same time (which, for now,
	# there isn't), this whole endeavor is thoroughly unnecessary.

cc @matthiasdiener