Experimental Pre-fork Pool: Recycle net ns (#890)

* fn: experimental prefork recycle and other improvements

*) Recycle and do not use same pool container again option.
*) Two state processing: initializing versus ready (start-kill).
*) Ready state is exempt from rate limiter.

* fn: experimental prefork pool multiple network support

In order to exceed 1023 container (bridge port) limit, add
multiple networks:

    for i in fn-net1 fn-net2 fn-net3 fn-net4
    do
            docker network create $i
    done

to Docker startup, (eg. dind preentry.sh), then provide this
to prefork pool using:

    export FN_EXPERIMENTAL_PREFORK_NETWORKS="fn-net1 fn-net2 fn-net3 fn-net4"

which should be able to spawn 1023 * 4 containers.

* fn: fixup tests for cfg move

* fn: add ipc and pid namespaces into prefork pooling

* fn: revert ipc and pid namespaces for now

Pid/Ipc opens up the function container to pause container.

api/agent/drivers/docker/docker_pool_test.go

@@ -8,13 +8,21 @@ import (
 	"github.com/fnproject/fn/api/agent/drivers"
 )
 
+func getDefaultCfg() *drivers.Config {
+	cfg := &drivers.Config{
+		PreForkImage: "busybox",
+		PreForkCmd:   "tail -f /dev/null",
+	}
+	return cfg
+}
+
 func TestRunnerDockerPool(t *testing.T) {
 	if runtime.GOOS != "linux" {
 		t.Skip("prefork only supported on Linux")
 		return
 	}
 
-	cfg := &drivers.Config{}
+	cfg := getDefaultCfg()
 
 	// shouldn't spin up a pool since cfg is empty
 	drv := NewDocker(*cfg)
@@ -95,7 +103,7 @@ func TestRunnerDockerPoolFaulty(t *testing.T) {
 		return
 	}
 
-	cfg := &drivers.Config{}
+	cfg := getDefaultCfg()
 
 	// shouldn't spin up a pool since cfg is empty
 	drv := NewDocker(*cfg)