Now that we've changed the ARM's kill() to not wait for outstanding batches, there should no longer be a need for "zombie cursors" or a "zombie cursor reaper" in mongos.

Note that we'll need to preserve "kill pending" cursors, for the case when killCursors is run an operation in the middle of a getMore. The operation will be interrupted, and until the cursor is checked back in (or out), the it will be marked as "kill pending."