QUIC: if client provides an STK which includes CachedNetworkParams, then

QUIC: if client provides an STK which includes CachedNetworkParams, then
store these and echo them back in future STKs if we have no newer
estimate to send. Flag protected behind disabled
FLAGS_quic_store_cached_network_params_from_chlo.

This is the followup to adding timestamp to the SourceAddressToken proto.
This allowed us to copy a previously provided CachedNetworkParams into
new STKs without losing timestamp data.

It also paves the way for recording BW estimate accuracy in the server:
we now have the client's provided BW estimate stored per connection, so
we can compare against BW that we see on the new connection (this will
be in a future CL).

Merge internal change: 76455340

R=rch@chromium.org

chromium: this code doesn't impact client side code because we don't
parse SourceAddressToken (we keep it as a string). For QUIC server,
we are not parsing optional cached_network_params. Added a TODO
to convert SourcAddressToken and CachedNetworkParams to protobuf.

[Ryan Hamilton, 2014-10-10 22:47:22]

lgtm