Move to proxy content store

[hinshun]

Currently, ipcs is implemented as a in-tree plugin for containerd, this comes with multiple problems:

• Need to recompile containerd for ipcs
• If the IPFS daemon is not listening when containerd starts then ipcs won't be initialized with a working client
• If the IPFS daemon restarts you need to recreate the client, though we could write a fault-tolerant client wrapper
• IPFS daemon includes a lot of subsystems not necessary to operate ipcs, increasing resource consumption
• We want to fine-tune and optimize distribution specific to container distribution, IPFS adds layers and complexity compared to just libp2p + bitswap

We should manage our lifecycle outside of containerd with a proxy content store.

• Implement a proxy content store
• Embed libp2p + bitswap instead of sidecar IPFS daemon
• Possibly wrap IPFS content store with boltdb metadata store like containerd