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.