cbbackupmgr: Should limit the number of vbuckets streamed from one node at a time.

Problem
Streaming multiple vbucket at once from one node, causes backups to be slower. As multiple vbucket will be backfilled at once, this will putting strain on the disk system and memory causing resource contention. In cases where the cbbackupmgr client is slow and the vbuckets are in the backfill state it can cause an increase of disk usage when compaction is triggered as two copies of the vbucket files will be kept, see MB-27538.

This problem has been seen before with cbbackup and was one of the reason cbbackupwrapper was created.

Suggests
It would be better to stream one vbucket at a time, the backup will be less consistence but that should not be an issue as today backups are not consistence across vbuckets.