New API: lwm2m_dm_execute() for a server to execute ressources on a client.
Remove unused variables.
New API: lwm2m_dm_create() and lwm2m_dm_delete() for a server to manage object instances on a client.
Code factorization.
Bug fix in registration.
Add parameters checks when sending or receiving an Execute.
Test: add a delete command in the server.
Use transaction callback mechanism when receiving all kinds of reply.