Implement proper symbolic tag negotiation for eager

[inducer]

• https://github.com/inducer/grudge/pull/222/files#diff-5fa6e6b713d37028c16f1bbc5a6d0b4547ecba0f86c8a52eb30e1ba1dc2614e4R496-R499
• Errors after switching to grudge@main illinois-ceesd/mirgecom#617 (comment)

cc @matthiasdiener

[MTCam]

Just a quick related note... the code here:

grudge/grudge/trace_pair.py

Line 508 in bde1ad7

tag_ub = actx.mpi_communicator.Get_attr(MPI.TAG_UB)

fails for array contexts that do not have the mpi_communicator attribute, but do pass a comm_tag. MIRGE-Com does this when it calls interior_trace_pairs, for example here:

https://github.com/illinois-ceesd/mirgecom/blob/452186f0365a5969e55693e07f87caeb712981d6/mirgecom/euler.py#L153

I'll spare you the generated error, but I've just a pushed a drive-by to fix the problem. #226

[inducer]

I've started putting some work towards this (#233) before realizing that this is all thoroughly pointless: Eager grudge can only have communication for one tag pending at the same time, otherwise it would deadlock. Therefore, all ranks must hit eager communication in the same order, and so we can do with these tags whatever we want; it won't matter until we allow multiple communications to be pending at the same time.