MB-32828: topkeys track default collection only
MB-32828: topkeys track default collection only

As per MB, the plan is to deprecate top-keys but for now only
track the default collection.

Commit ensures topkeys is only updated for the default collection
keys and we only use the logical key (collection prefix removed).

Tested on cluster_run

Change-Id: Idc36a0d0aa81986d0ee227356fcc437acf478ff9
Reviewed-on: http://review.couchbase.org/124015
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine