fix regression on deregistration + refactoring
free resource (server-sent event) when controler is destroyed
Avoid blinking at client list initialization.
Merge remote-tracking branch 'origin/master' into observe

Conflicts:
	leshan-core/src/main/java/leshan/server/lwm2m/client/Client.java
	leshan-core/src/test/java/leshan/server/lwm2m/message/californium/CaliforniumBasedRequestHandlerTest.java
	leshan-standalone/src/main/resources/webapp/js/controllers.js