Fix a race condition in Connection.isExpired().

This was causing some SPDY connections to be evicted early,
and preventing connection reuse.

The spdyConnectionReuse test becomes flaky with the
fix that makes MockWebServer use different sequenceNumbers
for different requests on the same socket. With the fix
the test is no longer flaky. The old test wasn't good enough
to detect what it wanted to detect!

This race has existed in the code since we added more
aggressive time-based expiration in January 2013. In my
basic tests the race impacted ~20% of connections on a
desktop VM. It may have been more on mobile.
Merge pull request #320 from square/jwilson/idle

Fix a race condition in Connection.isExpired().