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.
13 jobs for master in 24 minutes and 43 seconds (queued for 2 seconds)
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