Rewrite plugin version checker.

This commit removes the old version checker that used the DBO resource
page and replaces it with a custom checker that uses the "legacy" Spigot
resource API. The Spigot API is much more lightweight and doesn't
require any sort of parsing.

The new version checker uses a simple cache, keeping version checks
fresh for up to one hour, reducing the need to go fishing on every op
login. The cache resets on restarts, though, but this is acceptable.

Note that no attempt has been made to ensure correctness on multiple,
consecutive invocations when the cache is stale. If a cache refresh is
initiated, all update checks invoked before the cache refresh has ended
will behave as if no update is available. This is acceptable, because
update checks are non-essential, the time frame is extremely narrow, and
the most common result of an update check is "no updates available",
since the amount of update checks made is vastly greater than the amount
of updates released.

changelog.md

@@ -14,6 +14,7 @@ These changes will (most likely) be included in the next version.
 - It is no longer necessary to have recurrent waves for an arena to work. MobArena automatically creates a "catch all" recurrent wave in case the arena session reaches a wave number that isn't covered by any other wave definitions.
 - Entities outside of the arena can no longer target players, pets, or monsters inside of the arena.
 - Tab completion for `/ma kick` and `/ma restore` now uses actual player names instead of display names.
+- MobArena's internal version checker has been rewritten. It now uses the resource API of Spigot instead of DBO. It's also a lot more lightweight and caches results for up to one hour.
 
 ## [0.104] - 2019-08-08
 - Extended and upgraded potions are now supported in the item syntax by prepending `long_` or `strong_` to the data portion of a potion item (e.g. `potion:strong_instant_heal:1` will yield a Potion of Healing II). Check the wiki for details.