Move C++ code out of C headers

The additions into the server.h header used C++ language which caused C
programs to fail to compile. Moved the implementation of the EMAverage
class into the private Server class in the server.hh header and exposed it
via functions in the server.h header. Also temporarily moved
almost_equal_server_scores into the public server.hh as there is no
service.hh header.

server/modules/routing/readwritesplit/rwsplitsession.cc

@@ -648,7 +648,7 @@ void RWSplitSession::clientReply(GWBUF* writebuf, DCB* backend_dcb)
     ResponseStat& stat = backend->response_stat();
     stat.query_ended();
     if (stat.is_valid() && (stat.sync_time_reached()
-                            || backend->server()->response_time->num_samples() == 0))
+                            || server_response_time_num_samples(backend->server()) == 0))
     {
         server_add_response_average(backend->server(),
                                     stat.average().secs(),