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/runner/runner.go

@@ -115,10 +115,17 @@ func (r *Runner) queueHandler() {
 	}
 }
 
+func (r *Runner) hasAsyncAvailableMemory() bool {
+	r.usedMemMutex.RLock()
+	defer r.usedMemMutex.RUnlock()
+	// reserve at least half of the memory for sync
+	return (r.availableMem/2)-r.usedMem > 0
+}
+
 func (r *Runner) checkRequiredMem(req uint64) bool {
 	r.usedMemMutex.RLock()
 	defer r.usedMemMutex.RUnlock()
-	return r.availableMem-r.usedMem/int64(req)*1024*1024 > 0
+	return (r.availableMem-r.usedMem)/int64(req)*1024*1024 > 0
 }
 
 func (r *Runner) addUsedMem(used int64) {
@@ -136,7 +143,7 @@ func (r *Runner) checkMemAndUse(req uint64) bool {
 
 	used := int64(req) * 1024 * 1024
 
-	if r.availableMem-r.usedMem/used < 0 {
+	if (r.availableMem-r.usedMem)/used < 0 {
 		return false
 	}