Define congestion collapse in terms of excess overhead.

[mattmathis]

Define congestion collapse in terms of excess overhead.

Rename the prior congestion collapse section "Implement full backoff"

Closes issue #45

[martinduke]

Editors:

1. This metric is very interesting, but wonder if it's really a requirement for loss recovery, not congestion control (granted, these two are often intertwined). This would put it out of scope.
2. Our proposal would be to revert this, though perhaps we are misunderstanding the intent.