Add server health information to the Server type

[Will Warren]

Implement stuff suggested in this MineCenter bug: https://bitbucket.org/phybros/minecenter/issue/7/add-additional-server-health-information

The functionality should basically be the same information returned by the Essentials plugin's "/gc" command

[Will Warren]

Some more ideas:

Create a new struct in the IDL file called ServerHealth and add a list of them to the Server struct.

ServerHealth definition:

struct ServerHealth {
    1: required string label, // e.g. Uptime, Memory Allocated
    2: string value,          // e.g. 1d 12h 34m 8s, 500MB
}

And then change the Server definition to:

struct Server {
        1: string name,
        2: list<OfflinePlayer> offlinePlayers,
        // ... other properties omitted

        /**
         * Information about the server's overall health; memory usage,
         * cpu usage, uptime, chunk info etc.
         *
         * @since 1.7
         */
        16: list<ServerHealth> health,
}

Doing it this way ensures that we aren't adding any new methods to the (already huge) SwiftApi interface. Each new method increases the size by about 100KB. New properties increase the size a lot less.

[Will Warren]

Also add chunks and entities to the World type. That would be useful info.

[Will Warren]

Pretty much completed in a681601