lxd/response: Modernize FileResponse
lxd/response: Modernize FileResponse

This adds proper streaming support using ReadSeeker and does away with
the special-case file cleanup logic in favor of each file having a
cleanup function.

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

stgraber/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd