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.go

@@ -27,14 +27,17 @@ type Server struct {
 	MQ              models.MessageQueue
 	AppListeners    []ifaces.AppListener
 	SpecialHandlers []ifaces.SpecialHandler
+
+	tasks chan runner.TaskRequest
 }
 
-func New(ds models.Datastore, mq models.MessageQueue, r *runner.Runner) *Server {
+func New(ds models.Datastore, mq models.MessageQueue, r *runner.Runner, tasks chan runner.TaskRequest) *Server {
 	Api = &Server{
+		Runner:    r,
 		Router:    gin.New(),
 		Datastore: ds,
 		MQ:        mq,
-		Runner:    r,
+		tasks:     tasks,
 	}
 	return Api
 }
@@ -80,7 +83,7 @@ func (s *Server) UseSpecialHandlers(ginC *gin.Context) error {
 		}
 	}
 	// now call the normal runner call
-	handleRequest(ginC, nil)
+	s.handleRequest(ginC, nil)
 	return nil
 }
 
@@ -90,7 +93,7 @@ func (s *Server) handleRunnerRequest(c *gin.Context) {
 		ctx, _ := common.LoggerWithFields(c, logrus.Fields{"call_id": task.ID})
 		return s.MQ.Push(ctx, task)
 	}
-	handleRequest(c, enqueue)
+	s.handleRequest(c, enqueue)
 }
 
 func (s *Server) handleTaskRequest(c *gin.Context) {