This brings the Windows port more in-par with Linux. We're now making sure all of the pollsets are going to return immediately before calling the shutdown callback.

In the case we're using a threadpool, depending on where the threads are waiting in completion queue's next, they'll get stuck on shutdown because we're only kicking one of them. Instead, let's cascade-kick the pollsets when we shutdown so that we make sure all of them are exitting properly.

Simplify grpc_event into something that can be non-heap allocated.
Deprecate grpc_event_finish.
Remove grpc_op_error - use an int as this is more idiomatic C style.

In the presence of garbage collectors, this helps ensure that completion
queues outlive the servers that depend upon them.

A previous fix to make close() occur later can cause socket reuse by servers to fail as previous sockets are left asynchronously open.

This change:
- adds a callback to TCP server shutdown to signal that the server is completely shutdown
- wait for that callback before destroying listeners in the server (and before destroying the server)
- handles fallout

A previous fix to make close() occur later can cause socket reuse by servers to fail as previous sockets are left asynchronously open.

This change:
- adds a callback to TCP server shutdown to signal that the server is completely shutdown
- wait for that callback before destroying listeners in the server (and before destroying the server)
- handles fallout

Make debugging a little easier, and remove an unnecessary assert

This allows us to safely asynchronously add FDs in the
possibly-promoting unary add case.

Also fix the unary add async path to properly handle more of the extra
cases it needs to handle.