Improve goroutine management with tombs

[Casey Marshall]

PacketObject.Traverse is a convenient visitor-pattern like interface, but it leaks goroutines if the receiver returns early without reading all the objects from the channel. Other functions such as ReadKeys and ReadValidKeys have a similar problem.

We can "defer slurpUpTheRestOfTheChannel(someChan)" but it's a waste to have that goroutine continue working when we're just throwing away its messages until it closes the channel and exits.

Gustavo's tombs could be a potential solution. http://blog.labix.org/2011/10/09/death-of-goroutines-under-control

[Casey Marshall (cmars)]

Replaced this traversal with a visitor-style pattern that supports early exit.