linux/shim: reduce memory overhead by using ttrpc

By replacing grpc with ttrpc, we can reduce total memory runtime
requirements and binary size. With minimal code changes, the shim can
now be controlled by the much lightweight protocol, reducing the total
memory required per container.

When reviewing this change, take particular notice of the generated shim
code.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

Protobuild.toml

@@ -32,9 +32,13 @@ plugins = ["grpc", "fieldpath"]
   "google/rpc/status.proto" = "github.com/containerd/containerd/protobuf/google/rpc"
 
 [[overrides]]
-
 prefixes = ["github.com/containerd/containerd/api/events"]
-plugins = ["fieldpath"]
+plugins = ["fieldpath"] # disable grpc for this package
+
+[[overrides]]
+# enable ttrpc and disable fieldpath and grpc for the shim
+prefixes = ["github.com/containerd/containerd/linux/shim/v1"]
+plugins = ["ttrpc"]
 
 # Aggregrate the API descriptors to lock down API changes.
 [[descriptors]]