MB-29184 CouchRocks: Use Phosphor to trace RocksDBKVStore::scan()
MB-29184 CouchRocks: Use Phosphor to trace RocksDBKVStore::scan()

It would be useful to measure the actual time spent in KVStore::scan()
during a Backfill under RocksDB to compare with Couchstore.
We already use Phosphor to trace in CouchKVStore::scan(), so we add a
Phosphor Trace Event in RocksDBKVStore::scan().

Change-Id: I14d2766a41345682eb543b910b0d26eca0281637
Reviewed-on: http://review.couchbase.org/92420
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Reviewed-by: Tim Bradgate <f1a26ee65f021cb7d82b83dfb09abf144d3eb64f@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

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