Add support for tiddlers collections using more memory.

By default a tiddlers collection could read each tiddler
in the collection from the store twice. This is done to
save memory in "normal" situations. For large servers
(like tiddlyspace.com) this can potentially slow things
down when there is plenty of memory available.

See http://cdent.tiddlyspace.com/TiddlyWeb%20Collections%20Memory