[IO] fail hard if the globalCompleter would drop a Future

Currently the globalCompleter would drop Futures which he is
unable to submit to its SQ because io_uring_submit returned -EBUSY,
signalizing a full CQ.
For now we fail hard if this hopefully unlikely condition occurs.

Additional small code changes:
* don't wrap assert in if constexpr(DEBUG)
* annotate Future completion in globalCompleter as unlikely
* use prepared Future count as unsigned
* remove redundant parenthesis
* introduce helper function for IoContext* tagging
* introduce Stats::record_io_submit_full_cq default parameter