Add Read-Composite operation

[sbernard31]

This aims to implement #957.

(This is work in progress, not in reviewable state or testable state)

[sbernard31]

I finished to write a first "working" version. This was mainly to detect all the impact on current code base.
This should be enough to play with it but this is clearly not in a clean state.

Now I need to polish this (implement some missing details, clean the code, add more validation, add javadocs, some renames ...)
ReadComposite using "/" is not implemented now and will be probably not in this PR.

The API looks like this :

// read device model number
ReadCompositeResponse response = server.send(registration,
        new ReadCompositeRequest(ContentFormat.SENML_CBOR, ContentFormat.SENML_CBOR, "/3/0/0", "/1"));

if (response.isSuccess()) {
    List<LwM2mNode> content = response.getContent();
    content.get(0); // response for "/3/0/0"
    content.get(1); // response for "/1"
    ...
}

[sbernard31]

@madhushreegc @eliasweingaertner I think this is in in a reviewable and testable state.
(If you want to look at the code I advice to check commit by commit this will be more understandable)

My next step for another PR will be to look at the WriteComposite to check if the ReadComposite design matches correctly with it.

[sbernard31]

Unfortunately, I need to update the code by mainly switching all the List<LwM2mPath> + List<LwM2mNode> by Map<LwM2mPath, LwM2mNode>.

This was mainly because to decode nodes from WriteComposite. In this case the list of List<LwM2mPath> is unknown and so the result must return Node+Path, I'm not sure I'm so clear ... (see first commit of #959 for more details)

This impacts the ReadCompositeResponse API and so now it looks like :

// read device model number
ReadCompositeResponse response = server.send(registration,
        new ReadCompositeRequest(ContentFormat.SENML_CBOR, ContentFormat.SENML_CBOR, "/3/0/0", "/1"));

if (response.isSuccess()) {
    LwM2mSingleResource resource = (LwM2mSingleResource) response.getContent("/3/0/0");
    LwM2mObject object = (LwM2mObject) response.getContent("/1");
    ...
}

[sbernard31]

Still not entirely satisfied by the API an maybe we should go to something like #981.