[service] Provide an helper to retrieve object by name

Use it where necessary
[supervision_service] Return 'true objects', not str or json encoded one

It is the middleware layer which is in charge of the serialization in a
'transmission friendly' format.
[bin/morse_inspector] No need to json.dumps from morse.pc output

It is the job of pymorse to decode data!
[middleware/socket] Handle properly message which size > 4096

Fix #393
[doc] Remove some reference to old github laas/morse repository

While here, make clear that git.openrobots.org is now a mirror.