Sort out repeatability of queries

[timmartin]

I suspect that queries are being run unnecessarily in situations like the following:

Query<Foo> whatever = new Query<Foo>(...);
Foo x = whatever.ElementAt(1);
Foo y = whatever.ElementAt(2);

This needs to be tested, and if necessary fixed.

[timmartin]

I've implemented simple caching now, it remains to be seen whether this is enough to cover all the cases.