stream/sack: turn SACK record list into rbtree

stream/sack: turn SACK record list into rbtree

Convert to rbtree from linked list. These ranges, of which there can
be multiple per packet, are fully controlled by an attacked. The
attacker could craft a stream of packet in such a way that the list
would grow very large. This would make inserts/removals very expensive,
as well as the list walk that is done and size calculation and pruning
operations.

The RBTREE makes inserts/removals much cheaper, at a slight overhead
for 'normal' operations and slightly higher per record memory use.