A previous fix for SPIGOT-6814 implemented a callback function for the
PDC implementation that could be set to actively define a chunk as
unsaved, allowing chunks that have not been mutated through block
changes to still require saving if the chunks pdc was mutated.
This implementation however would pass a callback that references the
chunk access internally, meaning the PDC now actively holds onto a
callback that holds a reference to the entire chunk.
Aditionally, this change also impacted the pdc for item metas and
entities for really no reason whatsoever.
This commit re-implements the fix by introducing a new child of the pdc
implementation that the chunk now uses as its pdc. This specific
implementation maintains a dirty flag that is set to `true` on any form
of mutation and set back to false by the chunk that owns the PDC
whenever the chunk itself is flag as no longer dirty.
* CraftBlockState#getWorldHandle() would previously run into a NPE when being invoked for a non-placed BlockState. It now returns null in this case.
* CraftChest#getInventory() would previously encounter this NPE when being called for a non-placed BlockState. It will now automatically forward the call to #getBlockInventory() when it is being called for either a non-placed BlockState, or during world generation.
* CraftStructureBlock#applyTo was able to run into a NPE when being called for non-placed BlockStates (this is for example called by #getSnapshotNBT()).
* Replaced all no-world-generation preconditions with a general 'ensureNoWorldGeneration' method.
* Removed a few superfluous #isPlaced() calls. If getWorldHandle() reutrns a World, this already implies that the BlockState is placed.
