layer store: Handle volatile and transient layer

We store information about the layers in two files, layers.json and
volatile-layers.json. This allows us to treat the two stores
differently, for example saving the volatile json with the NoSync
option, which is faster (but less robust).

In normal mode we store only layers for the containers that are marked
volatile (`--rm`), as these are not expected to persist anyway. This way
informantion about such containers are faster to save, and in case
of an unclean shutdown we only risk loss of information about other
such volatile containers.

In transient mode all container layers (but not image layers) are
stored in the volatile json, and additionally it is stored in tmpfs.
This improving performance as well as automatically making sure no
container layers are persisted across boots.

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