Tasks which run on the MigrationUtilExecutor must not wait for shutdown to complete

All the blocking range deletion recovery work happens on the MigrationUtilExecutor and eventually calls into retryIdempotentWorkAsPrimaryUntilSuccessOrStepdown, which is a strictly blocking call that relies on globalInShutdownDeprecated/waitForShutdown.

Given that we join the MigrationUtilExecutor on shutdown, this can result in a deadlock where the shutdown thread is waiting for the executor to complete and the executor is waiting on shutdown to complete.