Feature/acksync response writer (#1267)

This implements a "detached" mechanism to get an ack from the runner
once it actually starts to run a function. In this scenario the response
returned back is just a 202 if we placed the function in a specific
time-frame. If we hit some errors or we fail to place the fn in time we
return back different errors.

api/models/call.go

@@ -14,6 +14,8 @@ const (
 	TypeSync = "sync"
 	// TypeAsync ...
 	TypeAsync = "async"
+	// TypeDetached is used for calls which return an ack to the caller as soon as the call starts
+	TypeDetached = "detached"
 )
 
 var possibleStatuses = [...]string{"delayed", "queued", "running", "success", "error", "cancelled"}