[SPDK] Preparation for SPDK 20.04 release

Hello all, The merge window for SPDK 20.04 release will close by April 24th. 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 '20.04' 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:%2220.04%22+status:open On April 24th new branch 'v20.04.x' will be created, and a patch on it will be tagged as release candidate. Then, on April 30th, a formal release will take place tagging the last patch on the branch as SPDK 20.04. Between release candidate and formal release, only critical fixes shall be backported to the 'v20.04.x' branch. Development can continue without disruptions on 'master' branch. PS. Due to rescheduling of US SPDK Summit, date for code freeze was moved to the original date. Thanks, Tomek