Move client implementations for content store and snapshotter

[dmcgowan]

The client implementations for content store and snapshotters are just proxy implementations of the interface. Move these next to the other implementations. These could be used for any proxy implementation.

[Random-Liu]

Will docker use this for all filesystem? Or only filesystem which containerd doesn't support today?

[dmcgowan]

Will docker use this for all filesystem? Or only filesystem which containerd doesn't support today?

The proposed snapshot proxy would only be used for snapshotters based on Docker graph drivers, which are not supported today. The proxy drivers could also be used to provided other implementations which all clients could end up using transparently, such as P2P content stores.

[Random-Liu]

@dmcgowan I see. Thanks! I'm just curious whether Docker will partially use containerd image management, and partially use graph driver. Or Docker will only use graph drivers for a long time.

[Random-Liu]

Sorry, I don't quite understand this PR. Why the package is called "proxy"?

Will Docker replace the grpc service implementation or the interface implementation?

[dmcgowan]

I'm just curious whether Docker will partially use containerd image management, and partially use graph driver. Or Docker will only use graph drivers for a long time.

The goal is to move Docker over in stages, ultimately completely using the image management and only graph drivers to support upgrade or graph drivers which do not have a snapshotter equivalent.

Why the package is called "proxy"?

These implementations of the content store and snapshotter just proxy over GRPC, there is no concrete implementation. I used the term "proxy" here because the caller is not intended to distinguish between using the proxy or directly using a concrete snapshotter.

[codecov-io]

Codecov Report

Merging #2341 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2341      +/-   ##
==========================================
+ Coverage   44.98%   44.99%   +<.01%     
==========================================
  Files          92       92              
  Lines        9340     9350      +10     
==========================================
+ Hits         4202     4207       +5     
- Misses       4459     4461       +2     
- Partials      679      682       +3

Flag	Coverage Δ
#linux	49.28% <ø> (ø)	⬆️
#windows	41.25% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
oci/spec.go	28.57% <0%> (-11.43%)	⬇️
metadata/buckets.go	56.79% <0%> (+2.12%)	⬆️

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[crosbymichael]

LGTM

[estesp]

LGTM