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/server_test.go

@@ -15,6 +15,7 @@ import (
 	"github.com/iron-io/functions/api/datastore"
 	"github.com/iron-io/functions/api/models"
 	"github.com/iron-io/functions/api/mqs"
+	"github.com/iron-io/functions/api/runner"
 	"github.com/iron-io/runner/common"
 )
 
@@ -84,10 +85,15 @@ func prepareBolt(t *testing.T) (models.Datastore, func()) {
 
 func TestFullStack(t *testing.T) {
 	buf := setLogBuffer()
-	ds, close := prepareBolt(t)
-	defer close()
+	ds, closeBolt := prepareBolt(t)
+	defer closeBolt()
 
-	s := New(ds, &mqs.Mock{}, testRunner(t))
+	tasks := make(chan runner.TaskRequest)
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	go runner.StartWorkers(ctx, testRunner(t), tasks)
+
+	s := New(ds, &mqs.Mock{}, testRunner(t), tasks)
 	router := testRouter(s)
 
 	for i, test := range []struct {
@@ -119,5 +125,4 @@ func TestFullStack(t *testing.T) {
 				i, test.expectedCode, rec.Code)
 		}
 	}
-
 }