MXS-1777: Add an EMAverage to the server struct, and a new slave selection criteria.

This is to support calculating the average from a session, and the slave selection criteria to be able to route based on averages. This commit, like the next one, have TODOs which you should feel free to comment on. Undecided things.

server/core/config.cc

@@ -3608,6 +3608,11 @@ int create_new_server(CONFIG_CONTEXT *obj)
             }
         }
 
+        // nantti, TODO. Decide whether to expose some of this in config, or if the values
+        // can be calculated at runtime. The "500" or sample_max affects how often a
+        // session should updates this stat. sample_max should be slightly lower than max sample
+        // rate (which is less than qps due to the noise filter).
+        server->response_time = new (std::nothrow) maxbase::EMAverage(0.04, 0.35, 500);
     }
     else
     {
@@ -4615,7 +4620,7 @@ bool get_suffixed_size(const char* value, uint64_t* dest)
 }
 
 bool config_parse_disk_space_threshold(MxsDiskSpaceThreshold* pDisk_space_threshold,
-                                     const char* zDisk_space_threshold)
+                                       const char* zDisk_space_threshold)
 {
     mxb_assert(pDisk_space_threshold);
     mxb_assert(zDisk_space_threshold);