MXS-2555 Simple Smart Router

Does the measurments, usage of the same  and persistence to file.
Kill is not implemented, so waits for all clusters to fully responds

The performance data uses a mutex and the persistence data file
is written while holding the mutex. This obviously needs to be
improved, but this commit shows the working concept.

server/modules/routing/smartrouter/CMakeLists.txt

@@ -1,6 +1,7 @@
 add_library(smartrouter SHARED
 smartrouter.cc
 smartsession.cc
+performance.cc
 )
 
 target_link_libraries(smartrouter maxscale-common mysqlcommon)