testdb: make dropall.js resilient to "not master"
Fix ErrNotFound bug just introduced on Iter change.
Trivial test fixes for 2.8.0-rc3.
Add Collection.NewIter method.

This is a refactoring of the first batch + cursor logic that is used
in the Pipe and Repair methods, so it may be used both internally in
other places and also externally when people depend on functionality
not yet implemented in the driver.
Support cursors on listIndexes and listCollections.