Add Transport Layer abstraction at Client side.

[sbernard31]

This aims to implement #1025 at Client side.

This is based on experimentation done in #1220.

The API looks like :

// You create your LeshanClient with the builder like before
LeshanClientBuilder builder = new LeshanClientBuilder("myclient");

// But builder does not allow to configure any details about transport layer anymore
// instead you need to provide a LwM2mClientEndpointsProvider
// LwM2mClientEndpointsProvider will provide 1 or several LwM2mClientEndpoint

// For Californium (coap/coaps), Endpoint Provider API looks like this :
// --------------------------------------------------------
CaliforniumClientEndpointsProvider.Builder endpointsBuilder = new CaliforniumClientEndpointsProvider.Builder(
		    new CoapClientProtocolProvider(), new CoapsClientProtocolProvider());

// Change Californium configuration :
Configuration californiumConfig = endpointsBuilder.createDefaultConfiguration();
californiumConfig.set(DtlsConfig.DTLS_RECOMMENDED_CIPHER_SUITES_ONLY, true);
endpointsBuilder.setConfiguration(californiumConfig);

// default is any LocalAddress.
endpointsBuilder.setClientAddress(InetAddress.getByName("192.168..0.1"));

// then add the endpoints provider to LeshanClientBuilder
builder.setEndpointsProvider(endpointsBuilder.build());

Some consequences / limitation of this changes:

• The way we handle observation is maybe too tied to Californium way.
• Identity should maybe used URI instead of InetSocketAddress
• ServerIdentity and Identity sounds to similar concept maybe ServerIdentity should be replaced by something like ServerProfile...
• We lost the API about choosing Request Encoding by ObjectEnabler, I don't know if this was really useful.
• the CLI option "-lp, --local-port of leshan-client-demo was removed as it does not make so much sense.

More tasks for later :

• there is not so much javadoc and so it should be added later.
• there is still some TODO TL : ... to implement.
• Currently we can add only 1 LwM2mEndpointsProvider but we should be able to add several one.
• of course code should also be moved to follow new Transport Layer Module design. #1295