feat(tvix): Implement all remaining RPC calls

Implement all remaining RPC calls on the RpcSstore client, remove a few
stub methods we had added that weren't actually present in the old
RemoteStore implementation, and add one more RPC call for getBuildLog
that is present in the store API, but that we hadn't added as a
stub *or* to the proto.

Fixes: #29
Change-Id: Id827f51a393ece4bc7bbecaf38aee9eb4b329770
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1692
Reviewed-by: kanepyork <rikingcoding@gmail.com>
Tested-by: BuildkiteCI

third_party/nix/src/proto/worker.proto

@@ -107,6 +107,9 @@ service WorkerService {
   // derivations that will be built, and the set of output paths that
   // will be substituted.
   rpc QueryMissing(StorePaths) returns (QueryMissingResponse);
+
+  // Return the build log of the specified store path, if available
+  rpc GetBuildLog(StorePath) returns (BuildLog);
 }
 
 enum HashType {
@@ -337,3 +340,7 @@ message QueryMissingResponse {
   uint64 download_size = 4;
   uint64 nar_size = 5;
 }
+
+message BuildLog {
+  string build_log = 1;
+}