Pause/freeze containers during commit

Initiates a pause before committing a container,
adds a pause option to the commit command, defaulting to 'true'.

Fixes #6267 
Fixes #3675

Docker-DCO-1.1-Signed-off-by: Eric Windisch <ewindisch@docker.com> (github: ewindisch)
it pauses, but it doesn't tell the processes to flush, or wait for a process in the middle of writing, or call sync, so it doesn't `prevent` it just reduces the risk?