daemon: paralellize disk usage computations
daemon: paralellize disk usage computations

Signed-off-by: Roman Volosatovs <de37379432a0923e9d62232e7753800cfd82d951@docker.com>

moby/moby,moul/docker,jameszhan/docker,yosifkit/docker,bearstech/moby,yosifkit/docker,mahak/docker,paralin/docker,docker/docker,nalind/docker,docker/docker,gnailuy/docker,mahak/docker,nawawi/docker,Ramzec/docker,moul/docker,cpuguy83/docker,vdemeester/docker,jimjag/docker,Mokolea/docker,nawawi/docker,konstruktoid/docker-upstream,nawawi/docker,Mokolea/docker,djs55/docker,bearstech/moby,jthelin/docker,flavio/docker,jimjag/docker,thaJeztah/docker,flavio/docker,konstruktoid/docker-upstream,djs55/docker,jimjag/docker,bearstech/moby,Ramzec/docker,thaJeztah/docker,thaJeztah/docker,cpuguy83/docker,jthelin/docker,moby/moby,thaJeztah/docker,trebonian/docker,jimjag/docker,gnailuy/docker,Mokolea/docker,paralin/docker,moby/moby,yosifkit/docker,nawawi/docker,flavio/docker,Ramzec/docker,gnailuy/docker,yosifkit/docker,moul/docker,moby/moby,Mokolea/docker,jameszhan/docker,docker/docker,konstruktoid/docker-upstream,nalind/docker,paralin/docker,cpuguy83/docker,vdemeester/docker,konstruktoid/docker-upstream,mahak/docker,bearstech/moby,moul/docker,trebonian/docker,djs55/docker,nalind/docker,trebonian/docker,jameszhan/docker,cpuguy83/docker,flavio/docker,gnailuy/docker,mahak/docker,nalind/docker,paralin/docker,trebonian/docker,vdemeester/docker,Ramzec/docker