Fix ServerEventListener deleting sessions right after exporting them
Add PlayerHolder.addAll(Player[])
Add online players to PlayerHolder in onEnable()
Add unregisterSerializer() method
Add @throws RuntimeException tag to registerSerializer()
Add SerializerObserver class
Use SerializerObserver for serializer config properties
Remove WaitingRoomObserver class
Remove serializer-enabled annotations and related
Register serializer only if it's enabled in the config
Return boolean instead of throwing Exception in (un)registerSerializer()
Unregister all serializers in stop()
Add private constructSerializer() method
Construct temporary serializer instance if not class isn't registered
Add private setSerializerEnabled() method
If serializer is disabled unserialize all online players