Log when a cursor is reaped for exceeding the timeout

Sometimes, queries fail because a cursor cannot be found. It would be really useful to see in the mongod logs when a cursor is reaped, to determine if that cursor was lost due to timeout.

As a part of this ticket, we should consider updating mongos to only log when a cursor is reaped (and not when it is merely marked for deletion, which is annoying for pinned cursors, which will not be timed out).