fn: runner status and docker load images (#1116)

* fn: runner status and docker load images

Introducing a function run for pure runner Status
calls. Previously, Status gRPC calls returned active
inflight request counts with the purpose of a simple
health checker. However this is not sufficient since
it does not show if agent or docker is healthy. With
this change, if pure runner is configured with a status
image, that image is executed through docker. The
call uses zero memory/cpu/tmpsize settings to ensure
resource tracker does not block it.

However, operators might not always have a docker
repository accessible/available for status image. Or
operators might not want the status to go over the
network. To allow such cases, and in general possibly
caching docker images, added a new environment variable
FN_DOCKER_LOAD_FILE. If this is set, fn-agent during
startup will load these images that were previously
saved with 'docker save' into docker.

api/agent/grpc/runner.proto

@@ -64,6 +64,14 @@ message RunnerMsg {
 
 message RunnerStatus {
     int32 active = 2;  // Number of currently inflight responses
+    bool failed = 3; // if status was successful or not
+    string id = 4; // call id if status image was used
+    string details = 5; // details for logging/debug
+    int32 errorCode = 6; // error code if not successful
+    string errorStr = 7; // error description if not successful
+    string createdAt = 8; // call latency details: initialization time
+    string startedAt = 9; // call latency details: start time in container
+    string completedAt = 10; // call latency details: end time
 }
 
 service RunnerProtocol {