Move taskrun() from SubmissionQueue to IoUring

[utkarshgupta137]

Currently, the taskrun() function is a method of SubmissionQueue, even though it doesn't rely on the head/tail of the submission queue. This makes the taskrun() flag less useful, because you've to do an extra load on the SubmissionQueue head while calling io_uring.submission(). This flag would be more useful, if it could be called from IoUring directly, without doing the extra load. I'm willing to submit a PR if interested.