Use CoAP transaction for deregistration

Client deregistration is now performed using a CoAP transaction.
It was previously done without, so no retry were performed, even
if the DELETE were a CON message.
Support for registration update

Registration update can be used for keeping NAT route open
For now the registration update nothing in the registration
parameters, it just send a PUT on the registration path.
Send the deregister message even if an update or registration is pending.

Signed-off-by: David Navarro <david.navarro@intel.com>
Merge branch 'master' into deregister-with-transaction

Signed-off-by: David Navarro <david.navarro@intel.com>

Conflicts:
	core/registration.c
Merge pull request #7 from eclipse/deregister-with-transaction

Use CoAP transaction for deregistration and implement registration update