Improve name generation on concurrent requests

Fixes #2586

This fixes a few races where the name generator asks if a name is free
but another container takes the name before it can be reserved.  This
solves this by generating the name and setting it.  If the set fails
with a non unique error then we try again.
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #6025 from crosbymichael/concurrent-names

Improve name generation on concurrent requests