GhostSync::percolate() should refresh dead cursors with OplogReader::tailCheck()

If a ClientCursor is dropped between batches (or during a batch because of an exception like kill op) the DBClientCursor for this ClientCursor will become "isDead()". If the DBClientCursor 'cursor' held by an OplogReader is dead, OplogReader::haveCursor() still returns true. In order to detect cursors, OplogReader::tailCheck() must be called. GhostSync::percolate() should call OplogReader::tailCheck() before using an old cursor in its oplog reader. Otherwise, percolate() may repeatedly attempt to read from a dead cursor and fail to see any results.