Fix trigger flowlet propagation

[reshadi]

Fix Trigger Flowlet logic

A few changes in this commit:
- Added a generic mechanism to add global flags, useful for when we want
  to add some temporary flags to test rollout of a fix/feature
- I noticed that when we wrap the callbacks, we always delete the
  triggerFlowlet of the top of the stack. The correct logic is that we
  should only do this if we have a `getTriggerFlowlet` function and may
  change the value in every invokation based on some argument of the
  function. Otherwise we should not touch the value.
- For UI event, we are not setting the callFlowlet correctly (based on
  how the callback was installed). This is mostly a noop since we don't
  use the callFlowlet of the UI event, but just wanted to make it more
  consistent.