[SPDK] Preparation for SPDK 21.10 release

Hello all,

The merge window for SPDK 21.10 release will close by October 22nd.
Please ensure all patches you believe should be included in the release are merged to 'master' branch by this date.
You can do it by adding a hashtag '21.10' in Gerrit on those patches.

The current set of patches that are tagged and need to be reviewed can be seen here:
https://review.spdk.io/gerrit/q/hashtag:%2221.10%22+status:open

On October 22nd new branch 'v21.10.x' will be created, and a patch on it will be tagged as release candidate.
Then, by October 29th, a formal release will take place tagging the last patch on the branch as SPDK 21.10.

Between release candidate and formal release, only critical fixes shall be backported to the 'v21.10.x' branch.
Development can continue without disruptions on 'master' branch.

Thanks,
Tomek