Allow querying other local server sessions through Plex API

Presently, you must send separate queries to every server (potentially several servers on the same LAN) to retrieve running sessions (On the /status/sessions API endpoint). The API allows locating all locally running servers through the /servers endpoint. Adding the ability to pass either:

a) An argument that automatically includes all sessions on all local servers (i.e, in the same JSON)
b) An argument containing the server ID you want the session list from

would be really handy for monitoring Plex activity. If added security is wanted, it could be required to include an appropriate auth token for the server you want the session list from.