Create a DAO AutoIndex or LazyIndex to only create Indices when first used

_From [k...@chromium.org](https://code.google.com/u/101260952816761938100/) on February 19, 2014 11:58:48_

Either create an AutoIndex which automatically adds indices when a new sort-order is selected or else create a LazyIndex decorator which defers the creation of an Index until it is first used. Alternatively, this could be built in to TreeIndex so that it doesn't build it's tree-index until it wins its first plan auction.

This feature would save memory in applications with a lot of indices (like QuickCompose).

_Original issue: http://code.google.com/p/foam-framework/issues/detail?id=203_