Feature request: manage local layer caches

[Desiki-high]

Before #140, the local layer caches were managed by contained service, nowadays we need to manage the layer caches ourselves.

Inspired by contained , we can add a bolt to record the changes of the local layers and clear them by LRU.

[Desiki-high]

GarbageCollect Design:

1. We need to know the latest use of each layer. Such as key: digest value: used, we can refactor LocalProvider.images.
2. GarbageCollect will delete the layer by digest one by one until the local cache size < cache_capacity.
3. When to garbage collect? We can't gc if any task is working, so we need to lock the LocalAdapter to prevent new tasks and wait for all tasks completed, then gc start.
4. Where to check local cache size. It seems LocalAdapter.Convert can do it.

Note: It seems gc also has a concurrency problem. This problem can be solved by gc func self or single flight.

cc @imeoer.

[Desiki-high]

Remaining work:

• Add the scheduled gc. gc: add scheduled gc task #179
• Separate gc and convert.chore: separate gc and convert #180
• Add doc for gc.docs: add garbage-collection.md for gc #183

cc @imeoer.

[Desiki-high]

@imeoer It looks like this job is done, let's close this issue and make a release.