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)
Merge pull request #8871 from tonistiigi/start-wait-hijack

Wait for hijack on docker start command