Allow GetAllVbSeqnos to support out of order execution

Secondary Index issues a GetAllVbSeqnos command to request sequence numbers for all vbuckets at bucket level. The GetAllVbSeqnos command can fail due to taking a long time to respond, if it is stuck behind many slow operations. If GetAllVbSeqnos had supported OoO then whilst the items are being fetched, in the background, GetAllVbSeqnos can execute without having to wait for the other to complete.