Router and filter instances cannot be destroyed before all worker
threads have exited. Otherwise there is a risk that data gets deleted
while someone might still be accessing it. Further, since all router
and filter instances are created by the main-thread it is better that
they are deleted by that thread as well (and not by whichever thread
happens to execute service_shutdown()). That will reduce the risk that
some unknown assumptions are violated.
The service header in include/maxscale/ contains the public part of the
service API. These functions can be safely used by the modules.
The internal header located in service/core/maxscale/ is used by the core
to initialize MaxScale at startup or to provide other services in a more
controlled way (the config_runtime, for example).
