#589 changed gracefullClose as follows:

1. With the runtime which provides the IO manager, STM race for packet arrival and timeout is introduced.
2. Otherwise, timeout is used.

I'm testing the approach 1) on my server. Unfortunately, CLOSE_WAIT happened again.
Since my server did not accept new connections, I'm sure that CLOSE_WAIT represents SYN packets in the listen queue.
This means that IO manager is blocked.

I need to understand why writeTVar is blocked.