lxd/network/acl/acl/load: Updates UsedBy to allow searching for multiple ACLs in one pass
lxd/network/acl/acl/load: Updates UsedBy to allow searching for multiple ACLs in one pass

 - Also passes the usage type record to the callback function as an `interface{}` so that type assertion can be used in the callback function.
 - Also reorders search to perform cheapest operations first, so that in firstOnly scenarios we can potentially do less work.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

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