Should increase the number of maps stored in vbucket_map_history to support delta recovery

We need to increase VBMAP_HISTORY_SIZE, in order to certify using more than 10 buckets. We could have different maps for these buckets , perhaps due to differing num_replicas, so we need the capability to store more than 10 past maps in vbucket_map_history in order to support delta recovery of buckets over 10 in number. We need to increase VBMAP_HISTORY_SIZE field or perhaps make it dynamic and/or configurable.

Alternatively, we can have a per bucket vbucket_map_history. 

http://src.couchbase.org/source/xref/vulcan/ns_server/src/ns_bucket.erl#804