Add ability to determine when a query is "complete"

[paralin]

Currently the result callback is called immediately, and every time the result changes, as the result is built. Instead, it should also be possible to mark a query as "fire once," and only get a result or error when the query is complete.

[paralin]

This is important now, will focus on this soon.

May add a @defer if waiting is default, or a @wait to mark sub-trees of a query as "wait until these are done before including them in the result."