functions: common concurrency stream for sync and async (#314)

* functions: add bounded concurrency

* functions: plug runners to sync and async interfaces

* functions: update documentation about the new env var

* functions: fix test flakiness

* functions: the runner is self-regulated, no need to set a number of runners

* functions: push the execution to the background on incoming requests

* functions: ensure async tasks are always on

* functions: add prioritization to tasks consumption

Ensure that Sync tasks are consumed before Async tasks. Also, fixes
termination races problems for free.

* functions: remove stale comments

* functions: improve mem availability calculation

* functions: parallel run for async tasks

* functions: check for memory availability before pulling async task

* functions: comment about rnr.hasAvailableMemory and sync.Cond

* functions: implement memory check for async runners using Cond vars

* functions: code grooming

- remove unnecessary goroutines
- fix stale docs
- reorganize import group

* Revert "functions: implement memory check for async runners using Cond vars"

This reverts commit 922e64032201a177c03ce6a46240925e3d35430d.

* Revert "functions: comment about rnr.hasAvailableMemory and sync.Cond"

This reverts commit 49ad7d52d341f12da9603b1a1df9d145871f0e0a.

* functions: set a minimum memory availability for sync

* functions: simplify the implementation by removing the priority queue

* functions: code grooming

- code deduplication
- review waitgroups Waits

api/server/routes_test.go

@@ -13,6 +13,8 @@ import (
 
 func TestRouteCreate(t *testing.T) {
 	buf := setLogBuffer()
+	tasks := mockTasksConduit()
+	defer close(tasks)
 
 	for i, test := range []struct {
 		mock          *datastore.Mock
@@ -34,7 +36,7 @@ func TestRouteCreate(t *testing.T) {
 		// success
 		{&datastore.Mock{}, "/v1/apps/a/routes", `{ "route": { "image": "iron/hello", "path": "/myroute" } }`, http.StatusCreated, nil},
 	} {
-		s := New(test.mock, &mqs.Mock{}, testRunner(t))
+		s := New(test.mock, &mqs.Mock{}, testRunner(t), tasks)
 		router := testRouter(s)
 
 		body := bytes.NewBuffer([]byte(test.body))
@@ -60,7 +62,10 @@ func TestRouteCreate(t *testing.T) {
 
 func TestRouteDelete(t *testing.T) {
 	buf := setLogBuffer()
-	s := New(&datastore.Mock{}, &mqs.Mock{}, testRunner(t))
+	tasks := mockTasksConduit()
+	defer close(tasks)
+
+	s := New(&datastore.Mock{}, &mqs.Mock{}, testRunner(t), tasks)
 	router := testRouter(s)
 
 	for i, test := range []struct {
@@ -94,7 +99,10 @@ func TestRouteDelete(t *testing.T) {
 
 func TestRouteList(t *testing.T) {
 	buf := setLogBuffer()
-	s := New(&datastore.Mock{}, &mqs.Mock{}, testRunner(t))
+	tasks := mockTasksConduit()
+	defer close(tasks)
+
+	s := New(&datastore.Mock{}, &mqs.Mock{}, testRunner(t), tasks)
 	router := testRouter(s)
 
 	for i, test := range []struct {
@@ -127,7 +135,10 @@ func TestRouteList(t *testing.T) {
 
 func TestRouteGet(t *testing.T) {
 	buf := setLogBuffer()
-	s := New(&datastore.Mock{}, &mqs.Mock{}, testRunner(t))
+	tasks := mockTasksConduit()
+	defer close(tasks)
+
+	s := New(&datastore.Mock{}, &mqs.Mock{}, testRunner(t), tasks)
 	router := testRouter(s)
 
 	for i, test := range []struct {
@@ -160,7 +171,10 @@ func TestRouteGet(t *testing.T) {
 
 func TestRouteUpdate(t *testing.T) {
 	buf := setLogBuffer()
-	s := New(&datastore.Mock{}, &mqs.Mock{}, testRunner(t))
+	tasks := mockTasksConduit()
+	defer close(tasks)
+
+	s := New(&datastore.Mock{}, &mqs.Mock{}, testRunner(t), tasks)
 	router := testRouter(s)
 
 	for i, test := range []struct {