Rapids shuffle - UCX active messages

[abellina]

JUCX is adding active messages to its API (openucx/ucx#6155). Active messages are a big change but they should remove the need for tag management, and bring also stability improvements (with some error handling already done for active messages).

This task is for the transport to change to use this, instead of tags, as a proof-of-concept at first, but document here findings and a likely PR once JUCX 1.11 is published.