Add null check to constructor, improve JavaDoc and log messages
Use standard property DEFAULT_COAP_PORT for configuring CoAP endpoint
port,
better use Californium Server's constructors in order to avoid
generation of default endpoint if not necessary, add ConnectorFactory to
OSGi bundle in order to support definition of a secure endpoint without
a direct dependency on Scandium
Remove obsolete optional import of Scandium
Update JavaDoc of ManagedServer
- Move ConnectorFactory to element-connector
- Introduce EndpointFactory for abstracting out the strategy for 
instantiating endpoints
- Expose a ManagedServer's endpoints by means of a
ServerEndpointRegistry
- JavaDoc improvements
- Add accessors for a server's endpoints to ServerInterface
- Make start() method of Endpoint throw an IOException if it cannot
be started
Add a property name constant for the default secure CoAP port to
NetworkConfigDefaults
Add test cases for retrieving endpoints from managed server
- refactor MessageIntercepter to MessageInterceptor
- do not add MessageLogger as default MessageInterceptor
to CoAPEndpoint