Reduce CPU associated with large skipped sequence queue

When the skipped sequence queue grows very large, the cost associated with removal from that queue becomes a performance bottleneck.  SkippedSequenceQueue.Remove does a sort.Search in the slice to find the value, then does a slice shift/copy to remove the element.  For large skipped sequence queues, the memory move associated with that shift becomes a performance bottleneck.

The current slice-based queue should be replaced with a list+map - the map for direct access to elements by sequence, and the list for memory efficient removal from an ordered list.