Ensure VCH creation log streaming flushes all data before exiting

[zjs]

See #6665 (comment):

Do we also want a Flush call that waits until all data written has been read? Currently I think it's possible for vic-machine to exit without having pushed all of the log data to the datastore, depending on timing.
I would not be surprised to see truncation of the last informational portions of the output.

And #6665 (comment):

The way this is written there's nothing preventing multiple concurrent calls to Read which would leave all but one thread blocked indefinitely.

I think both Write and Close should be using cond.Broadcast(). If we're being good then Read() should also have a deferred Broadcast so that one Read completing will allow another to continue - this model also allows for addition of a Flush semantic.

And #6665 (comment):

There is a race here - we could have called bp.Close() before the bp.Read takes place. I'm not sure it can be resolved and still have the test pass reliably without changing Close to a flushing semantic, i.e. Closed to writing, but allows read to continue to drain existing data.

[andrewtchin]

this is a ship stopper per hagen