* use hash of comm_tag if not numeric

* Drop comm_tag_to_mpi_tag

* modulo tag_ub

* flake8

* use actx mpi_comm

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* User persistent_dict hashing to guess a tag

Co-authored-by: Andreas Kloeckner <inform@tiker.net>