Use `unsigned long long` for SDP session id and version
instead of unsigned int.
Refactor all usage of them accordingly.
Additionally intrdouce new utils functions for conversion:
- `ulonglong2str()` - converts `unsigned long long` to `string`
- `str2ull()` - converts `string` to `unsigned long long`
Change-Id: I4210349a5442d4173b14227497f4a01d68cad7a4