`keep-exp` enabled should not remove XP when entering the arena

[Josh65-2201]

Feature request

Short description

As I use XP on my server as currency (using the Vault plugin) when it gets removed by the arena it removes all currency from players. While the keep-exp adds it back after, it doesn't detect this happened as an event and resets the players currency until they gain XP again witch is annoying and could be lost completely.

Implementation details

Either follow the keep-exp flag or make a new remove-exp-on-start flag

Additional info

[garbagemule]

It looks like keep-exp isn't actually documented on the wiki, which is a bit surprising considering how old this setting is. It was removed at some point, and then re-introduced back in 0.103, but it's behavior has always been the same: it allows experience collected during the arena session to be added back to the player's experience pool after restoring the experience the player joined with. It is technically a type of "silent reward" that's granted along with other rewards when the player leaves the arena. The issue described here has nothing to do with the keep-exp setting.

I don't think introducing a weird workaround in MobArena's join/leave procedure is the right approach here. The point of the procedure is to create a predictable snapshot flow that allows MobArena to behave as a minigame rather than something that has a bunch of unintended side effects.

The experience snapshot works by recording the player's total experience, level, and current experience (apparently all three were necessary once, not sure if they still are) and zeroing them out on join and restoring them using the exact same methods on leave. If whatever plugin that manages the experience-as-currency behavior can pick up the zeroing, it should be perfectly capable of picking up the restoration as well, since the triggers are the same. I think it's worth figuring out what's happening in the experience-as-currency plugin first and try and see if there's an underlying issue to solve somewhere else before jumping to duct tape boolean flags that may or may not fix the inconsistent symptoms we're seeing.