Don't close the SPDY peer too early.

This was occasionally causing tests to fail because the test was
observing the connection close.
Merge pull request #1116 from square/jwilson_1102_peer_close

Don't close the SPDY peer too early.
Attempt to improve MockWebServer reliability.

Use the same InetAddress when binding to a port as when offering the
MockWebServer's address.

Wait for the executors to all shutdown before returning from shutdown().