This reverts commit 86af5ce0.

Unfortunately we have two scopes top and task that may conflict when
esi_level is zero. This restores the id field, making struct vrt_priv
slightly larger than when it used to be managed as a tailq. But in
the context of a red-black tree for dynamic priv lookups the benefits
should still outweigh the slight increase in memory footprint.

Conflicts:
	bin/varnishd/cache/cache_vrt_priv.c

Fixes #3003