Add three methods for lease service so that the client can use it to
manage the resource by lease, not just gc.root label. With the following
methods, it is easy for client to maintain their own cache system.
```
- AddResource(context.Context, Lease, Resource) error
- RemoveResource(context.Context, Lease, Resource) error
- ListResources(context.Context, Lease) ([]Resource, error)
```
And the resource is to be
```golang
type Resource {
ID string
Type string
}
```
For the snapshots, the Type field will be formatted by
snapshots/%{type}, like snapshots/overlayfs.
fix: #3295
Signed-off-by: Wei Fu <fuweid89@gmail.com>
Because of a side-effect import, we have the possibility of pulling in
several unnecessary packages that are used by the plugin and not at
runtime to implement protobuf structures. Setting these imports to
`weak` prevents this from happening, reducing the total import set,
reducing memory usage and binary size.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
