Use OplogObserveDriver for most selectors.

Previously OplogObserveDriver was only used for selectors which
performed equality checks against scalars. Now that we believe minimongo
to be more robust in the face of more MongoDB edge cases, we use
OplogObserveDriver (if configured) for any selector that minimongo can
compile except those containing $near or $where.

(We still do not use OplogObserveDriver for cursors with skip or limit.)