Fix Essentials#getUser not caching newly created NPC users (#5129)

Essentials/src/main/java/com/earth2me/essentials/Essentials.java

@@ -1074,6 +1074,11 @@ public class Essentials extends JavaPlugin implements net.ess3.api.IEssentials {
                 LOGGER.log(Level.INFO, "Constructing new userfile from base player " + base.getName());
             }
             user = userMap.loadUncachedUser(base);
+
+            // The above method will end up creating a new user, but it will not be added to the cache.
+            // Since we already call UserMap#getUser() above, we are already okay with adding the user to the cache,
+            // so we need to manually add the user to the cache in order to avoid a memory leak and maintain behavior.
+            userMap.addCachedUser(user);
         } else {
             user.update(base);
         }

Essentials/src/main/java/com/earth2me/essentials/userstorage/ModernUserMap.java

@@ -149,6 +149,10 @@ public class ModernUserMap extends CacheLoader<UUID, User> implements IUserMap {
         return null;
     }
 
+    public void addCachedUser(final User user) {
+        userCache.put(user.getUUID(), user);
+    }
+
     @Override
     public Map<String, UUID> getNameCache() {
         return uuidCache.getNameCache();