Refactor QuicConnection's async write behavior to record a packet as

Refactor QuicConnection's async write behavior to record a packet as
sent as soon as the write starts, not when it completes.

In QuicStreamFactoryTest, MockClock's now() is returning 0 and that
was causing a DFATAL in QuicUnackedPacketMap::GetLastPacketSentTime().
Noticed, in QuicConnection unittests, we are advancing clock by 1 second
and made a similar change to QuicStreamFactoryTest's MockClock.

Merge internal change: 73648585

R=rch@chromium.org

[Ryan Hamilton, 2014-08-21 23:02:54]

lgtm