Optimize Mumble Link identity JSON parsing

[Archomeda]

The identity is currently cached, but further improvements can be done:

• Only a maximum of one check per tick has to be done to see if the identity was updated compared to the previous update; currently it compares the identity strings every time, regardless of it has been done already in the same tick
• The .ToString() call of the race allocates a new string every time, even if it's the same. A static lookup dictionary can be a solution