Previously we stored the esi-metadata in the object workspace, but since we
are trying to make that a snug fit and we cannot preestimate how much space
ESI parsing will need, this no longer works.

Instead parse the ESI metadata into the workers workspace and when
done, allocate a storage object and move it all into that.

Beware that this may increase the memory cost for ESI objects by the stevedores
granularity.

Fixes #578



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4351 d4fa192b-c00b-0410-8231-f00ffab90ce4