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/driver.go

@@ -197,6 +197,8 @@ type Config struct {
 	PreForkPoolSize uint64 `json:"pre_fork_pool_size"`
 	PreForkImage    string `json:"pre_fork_image"`
 	PreForkCmd      string `json:"pre_fork_cmd"`
+	PreForkUseOnce  uint64 `json:"pre_fork_use_once"`
+	PreForkNetworks string `json:"pre_fork_networks"`
 }
 
 func average(samples []Stat) (Stat, bool) {