MB-100 traverse sample items belong to a single shard

This fix may not need after integrating multi-dispatcher implementation
(global io manager). Otherwise, without the fix, the unit test
intermittently runs into time out error for following scenario.

for iteration over vbid = 0 ... vbid = 99
bgfetch for vbid = 0; shard 1/2/3 go to sleep
bgfetch for vbid = 1; shard 0/2/3 go to sleep, wakes up shard 1
bgfetch for vbid = 2; shard 0/1/3 go to sleep, wakes up shard 2
.
.
.
repeating upto vbid = 99, while the single dispatcher interleaving
among 4 individual bgfetch deamon tasks, which further slows down
servicing requested bgfetch jobs

Change-Id: I364eae9e8b3f7aaf9c69b4a148189ea0f289044c