use more accurate criteria for rebalance progress

1. filter out all NaN values
2. filter our all zero values
3. take first and last indexes

Change-Id: I66af6243bf45f7fa8b7707f11f4f0d522c2092ce
Reviewed-on: http://review.couchbase.org/30862
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>