Wait for hijack on docker start command

With current implementation there was a possibility
that /start responds quicker than /attach, meaning that
some output would be clipped.

Fixed so the implementation matches with `docker run`.

This also fixes the flaky test results for TestCreateEchoStdout.

Signed-off-by: Tõnis Tiigi <tonistiigi@gmail.com> (github: tonistiigi)

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More info about flaky TestCreateEchoStdout:
Seems I wasn't the only one who got it: https://www.google.com/search?q=TestCreateEchoStdout
Daemon log difference for fail/pass: https://www.dropbox.com/s/abcmfoqen4rsxyo/Screen%20Shot%202014-10-31.png