Support simultaneous unpacking of same layer
Support simultaneous unpacking of same layer

Prevent unpack failures due to id collision or the need to
wait for another process to finish unpacking. Always attempt
to unpack a layer and handle collisions on commit. Commit
collisions are easily handled as it could be considered the
same as a successful unpack.

Signed-off-by: Derek McGowan <e1c79a582b6629e6b39e9679f4bb964d25db4aa8@mcgstyle.net>

miaoyq/containerd,vdemeester/containerd,kunalkushwaha/containerd,dmcgowan/containerd,thaJeztah/containerd,thaJeztah/containerd,kunalkushwaha/containerd,mikebrow/containerd,vdemeester/containerd,containerd/containerd,crosbymichael/containerd,miaoyq/containerd,estesp/containerd,AkihiroSuda/containerd,crosbymichael/containerd,mikebrow/containerd,dmcgowan/containerd,estesp/containerd,containerd/containerd