VBucketBackupWriter should be able to 'CloseAllVBuckets' concurrently

After mistakenly running performance testing against the wrong version (meaning it was using DCP V1 not V2 e.g. 1024 vBuckets at once), I noticed that backup performance wasn't as good as it should have been. After looking at the logs it's clear that the backup itself was faster, however, we spent a considerable amount of time waiting for the vBuckets to close. Since the new VBucketBackupWriter was designed to handle sequential vBucket backups (for cloud) I didn't consider that in cases where we are backing up against pre 6.5.x/CK/CC clusters we would be closing all the vBuckets at once. We should make it so that when closing more than 'n' vBuckets we do it concurrently.