Make stage 2 chunk unload utilise I/O future write

When the I/O scheduler was rewritten, it was designed to
take writes as futures (i.e. CallbackCompletable, Completable).
We can utilise this functionality in entity and chunk unloading.

Provided that the write is scheduled during stage 3, we can remove
the unload tasks - even if the write value is not yet ready.

For entity chunk unloading, when the entity chunk is transient it needs
to load the current data on-disk to save. We can create a write future
which depends on the result from loading the current data from disk.
The previous behavior was to perform a blocking read on the chunk data,
as the previous behavior was designed with an I/O scheduler which
could not perform future writes.

For chunk unloading, the logic already issues a future value write.
The only changes made are to forward exceptions to the I/O thread
and to move the unload task completion to the stage 2 function
itself (which additionally avoids a double write schedule).

With these changes, all of chunk, entity, and poi tasks issue
an I/O write during unload (unless there is nothing to write). This
allows for all of the unload tasks to be removed during stage 3.

Additionally, make the I/O scheduler properly handle exceptions from
future writes - print the exception and avoid the write. Previously,
the behavior was to not print an exception and to delete the data
from disk.

The main benefit from these changes is that transient entity chunk
unloads no longer issue a blocking read from disk.

Tuinity/Moonrise#144
Tuinity/Moonrise#146

Author: Spottedleaf