Add congestion control states to sender.
Add congestion control states to sender.

This change just introduces different congestion control states and
ensures the sender.state is updated to reflect the current state
of the connection.

It is not used for any decisions yet but this is required before
algorithms like Eiffel/PRR can be implemented.

Fixes #394

PiperOrigin-RevId: 262638292

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor