Improve Event Callback Reentrancy

[nibanks]

This makes a number of changes in an attempt to improve reentrancy around MsQuic event callbacks to the app:

1. The connection now tracks if it is in a reentrant API call (e.g. the app is calling a blocking function on the MsQuic thread).
2. A couple API calls were made async, even if on the callback thread (StreamShutdown and StreamStart(async)).
3. We now debug assert if we ever try to indicate an event to the app if we are in a non-*Close reentrant API call.