Stats: Reduce calls to new() in server_stats
Stats: Reduce calls to new() in server_stats

Use 'const char*' for stat labels (i.e. constant strings) instead of
std::string, to avoid the cost of allocating memory at runtime (at
least with GCC 4.9 on Linux).

Change-Id: I8ca460ec4dcf231da9d6f44c2c171a0c090e5615
Reviewed-on: http://review.couchbase.org/55042
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>

owendCB/memcached,daverigby/kv_engine,owendCB/memcached,daverigby/kv_engine,daverigby/memcached,couchbase/memcached,daverigby/memcached,couchbase/memcached,daverigby/kv_engine,daverigby/memcached,daverigby/kv_engine,owendCB/memcached,couchbase/memcached,owendCB/memcached,couchbase/memcached,daverigby/memcached