memory utilization on slave can be higher than the master

When verifying the fix for MB-1843, it's been found that a single master, two slave setup (all vbucket 0, using only vbucketmigrator, not full membase cluster) one slave has a larger amount of memory utilization than another.

From the reporter:
> One of the slave memory is going up by 16% (7.1GB instead of 6.0GB) when the
> replication happens. I have tried this across different set of machines and
> found the same behavior.

The specific numbers are 8.3GB for the master with 5.9GB for the master with 7.1GB for the first slave and 5.9GB for the second slave.