Expire UserMap cache more aggressively and add cache debugging

[pop4959]

Fix #5327

There are a couple of issues with the usercache, namely:

• With a large / oversized heap, the maximum number of users that can be stored in the cache easily exceeds the number of total users. This generally means that the cache can and will store every single user on the server, which is usually not desirable (this defeats the purpose of using a cache).
• In the linked issue, a 24 GB heap is being used, which means Essentials allows 25,769,803,776 / 1024 / 96 = 262,144 cache entries. The server in the issue only has ~ 63,000 users, but all 63,000 were being loaded resulting in almost 1 GB memory usage from Essentials which is unacceptable.
• There is no expiry of values in the cache, even after not being used for a long time. This is a problem, as memory will start to leak anyways, especially when the maximum cache size is very large or effectively unbounded as mentioned above.

Solution:

• More aggressively limit the maximum cache size. I'm not even sure if we want to use max memory size in the calculation here, but for lack of a better alternative that scales, I changed max memory / 1024 / 96 to max memory / 1024 / 1024.
• Given the linked issue again, this would mean a maximum cache size of 24,576 < 63,000 meaning we will no longer ever store every single user. And that's with an oversized heap, for a normal server it will be far less and more reasonable. For a 1 GB heap the cache can store around 1000 users which is probably plenty for a server that size.
• Expire values in the cache that are not being accessed. I've set the default to 10 minutes, as that seems plenty (if the user is not being accessed at all in 10 minutes they're probably not going to be accessed again for a while). We could be more aggressive with this, but I think this is good enough for now. It solves the main concern of values potentially never expiring, and shouldn't really negatively impact performance with supported usage.

Worth mentioning is that in the linked issue, for some reason, all ~63,000 users were being cached. This should not happen normally with typical usage of the cache, and implies that (probably some plugin) something is loading every user into the cache. To help debug this I've added another system property to log every time something gets added to the cache.

bad