This changes the progress printer's pause and unpause implementation to
be reentrant to prevent race conditions and it also allows the status
updates to be buffered when the display is paused.
The previous implementation mixed the pause implementation with the
finish implementation and could cause a send on closed channel panic
because it could close the status channel before it had finished being
used. Now, the status channel is not closed.
When the display is enabled, the status channel will be forwarded to an
internal channel that is used to display the updates. When the display
is paused, the status channel will have the statuses buffered in memory
to be sent when the progress display is resumed.
The `Unpause` method has also been renamed to `Resume`.
Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>
Jonathan A. Sternberg
34e59ca1bd