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
Registration-Update-Fix

Conflicts:
	leshan-core/src/main/java/leshan/server/lwm2m/client/Client.java
	leshan-core/src/main/java/leshan/server/lwm2m/client/ClientRegistryImpl.java