Prevents the copy method from calling discard on the
writer when the reader is not seekable. Instead,
the copy method will discard up to the offset.
Truncate is a more expensive operation since any
bytes that are truncated already have their hash calculated
and are stored on disk in the backend. Re-writing bytes
which were truncated requires transfering the data over
GRPC again and re-computing the hash up to the point of
truncation.
Signed-off-by: Derek McGowan <derek@mcgstyle.net>
