epoch system loom copies should be reflinks when possible

[barter-simsum]

a number of filesystems support reflinks which are CoW copies of files that
share common blocks (and initially take up zero space) and only use disk to
store differing blocks.

https://btrfs.readthedocs.io/en/latest/Reflink.html

• macos apfs
• linux xfs, btrfs, others

we should use reflinks when the backing filesystem supports it to reduce disk
usage of the epoch system.