Introduce a set of performance tweaks.

The most notable changes are a refactor of DataManager to make the caching system easier to tweak, and the ability to use Lua scripts that run on the Redis server to optimize certain batch operations (currently, only getServersToPlayers() uses this).

This also paves the way for me to deprecate old versions of Redis. Anything older than 2.6 is ancient, and even 2.6 is ancient, so it ends up being a net overall benefit.

src/main/resources/lua/server_to_players.lua

@@ -0,0 +1,20 @@
+-- This script needs all active proxies available specified as args.
+local serverToData = {}
+
+for _, proxy in ipairs(ARGV) do
+    local players = redis.call("SMEMBERS", "proxy:" .. proxy .. ":usersOnline")
+    for _, player in ipairs(players) do
+        local server = redis.call("HGET", "player:" .. player, "server")
+        if server then
+            if serverToData[server] then
+                local data = serverToData[server]
+                data[#data + 1] = player
+            else
+                serverToData[server] = {player}
+            end
+        end
+    end
+end
+
+-- Redis can't map a Lua table back, so we have to send it as JSON.
+return cjson.encode(serverToData)