Add container layer store

When in transient store mode we have two writable layer stores, the
regular one (`overlay-layers`) and a transient one for container
layers (`overlay-transientlayers`). The later is always nil in the
regular mode and is then used for container layers too.

The reason for this split is that we want to be able to pull and
create persistent image layers that are persistent, but then use
transient storage for the `layers.json` of the container layers. That
means "podman run" only writes to tmpfs files when in transient store
mode, which is a lot faster.

Note: The actual storage for the layers is still on persistant
storage, so tmpfs use will not fill up memory. However, this means
that you want to clean up the `overlay-transientlayers` diretory on
boot to avoid leftover directories from using diskspace.

Signed-off-by: Alexander Larsson <[email protected]>