scary mongos messaging in rc2 related to unsharded cursors

In 2.6, we directly check both the sharded and unsharded cursor cache for cursor ids - only one of these can succeed. However we message OCCASIONALLY with:

Sharded CursorCache missing cursor id: 2354118951366269003

which is misleading, since generally this just means the cursor is unsharded.

Also we rely on a noisy and inefficient function to determine if the getMore'd collection is sharded, which can be replaced.