PROCESS_COMPLETIONS_ON_SUBMIT: acquire CQ before submission

[problame]

Before this PR, it could happen that the immediate completion was processed by the poller.
With this PR, it is guaranteed to be processed by the submitter.

TODOs:

• the comment added in this PR mentions a TODO, should understand whether the poller still gets woken up (=> use tracing_chrome to validate?)
• do a benchmark run; the nop benchmarks shows a few percent improvement and less variance with a single submitter, which is promising