Rework the way GRUU support is implemented.

* define two functions in RegistrarDb to convert a sip.instance to gr parameter and vice versa, used everywhere needed
* set the pub-gruu into the ExtendedContact, instead of synthetize when generating the 200 Ok response
* Fix an issue where the null character was accidentally inserted at the end of a c++ string, causing an ostringstream to stop (when printing ExtendedContact's paths).
* Eliminate useless code.

The initial goal of this rework is to allow a SIP client to REGISTER with a GRUU address as contact.
This wasn't possible because Flexisip was confusing the gr parameter it was computing and the gr parameter set by the client in the Contact header.