Fix hidden bug when entity properties are updated via ConfigEntityBase::set()

As described in #3542808-5: Add variants of plugin getters to our entity classes that don’t throw exceptions → , there is a hidden bug where updating properties of our index/server entities via their set() methods (inherited from ConfigEntityBase) could leave them in an inconsistent state, as that method won’t properly reset the corresponding cached plugins or server entity (e.g., $this->processorInstances when setting $this->processor_settings).

Unfortunately, the set() method is used by EntityForm so it is more a coincidence that this hasn’t led to any actual bugs in the module so far. (There could be such bugs in related modules, though.)

We should make sure that changing properties via set() never leaves the entity in an inconsistent state.