blob gc feature for fscache with ``inuse`` and ``cull``

[hsiangkao]

We should implement these to avoid too many unused cachefiles.

also see, https://github.com/lostjeffle/demand-read-cachefilesd/blob/main/test/test13-daemon.c

[hsiangkao]

@nan1994 @imeoer please help discuss these interfaces in order to make the gc functionality work.

[adamqqqplay]

It is also discussed in https://github.com/containerd/nydus-snapshotter/pull/262