Embed environment in CRDT objects

CRDT objects should embed a reference to an Environment and use it to get timestamps, instead of letting the user provide a timestamp on each API call.

• dependency inversion (DP strategy)
• proper encapsulation: delegate CRDT-type specific operations to CRDTlib
• avoid useless code duplication
• automate timestamps management, even without using the c-service → avoid errors (re-use of an old timestamp…)