Cleanup all singletons

The problem with starting Tango twice in one process basically boils down to us not deleting and nulling singleton objects. I find the following singletons:

> rg "\b_instance\b" src/include/
src/include/tango/server/utils.h
128://            protected and the _instance data member is static)
1176:    TANGO_IMP static Util *_instance;

src/include/tango/server/eventsupplier.h
286:    static NotifdEventSupplier *_instance;
393:    static ZmqEventSupplier *_instance;

src/include/tango/server/dserverclass.h
524://            protected and the _instance data member is static)
545:    TANGO_IMP static DServerClass *_instance;

src/include/tango/server/dserversignal.h
166:    static std::unique_ptr<DServerSignal> _instance;

src/include/tango/client/ApiUtil.h
342:    static ApiUtil *_instance;

We are already cleaning up the DServerSignal singleton in server_cleanup() which is called in server_cleanup(). And we already provide a function so that users can clean up the ApiUtil singleton.

For the other singletons (Util, NotifdEventSupplier/ZmqEventSupplier[1], DServerClass) I have added cleanup() static method and for all except Util I am calling the cleanup at (what I think is) an appropriate time during Util::shutdown_ds(). For Util it is up to the user to call Util::cleanup() themselves if they want to re-create the Util singleton.

TODO

• Wait for !1501 (merged) and add some tests here to check that e.g. the event system is working etc.

• Double check all static data is getting reset appropriately

• Double check that the order things are being cleaned up in makes sense

• Test this against the pytango unit tests

  Seems to work for simple cases, but they cannot change the green_mode between runs. @yamatveyev Suggests this is acceptable for now.

• Try out ASAN to see if I have fixed the problem in event_lock

  Save this for another time as we want to get this into release/10.3.0

• Target 10.3.0 release branch

• Get it working on Windows Skip tests on Windows

[1] Interestingly, the EventConsumers are just regular members of the ApiUtil singleton. I don't know if we want to look at making EventSuppliers and EventConsumers to follow the same pattern.

Closes #1556 (closed).