Unify critical session running in hls

[soulomoon]

There are some places we serialize critical session.

• Reactor thread, running the notification handler, by running it in the reactor thread.
• sessionLoader, we load session one by one, spawn a thread each time, and wait for the preivous thread to end.
• sessionRestart, we runs it in seperate threads but using an MVar to do the serlization.

The latter two create a challenge for our shutdown handler to halt the hls. Since they are not cancelable. A possible solution, would be to switch the latter two to the first approach. Create a thread specifically to run the target critical session.

[soulomoon]

This might fix the hanging problem happend in windows https://github.com/haskell/haskell-language-server/pull/2497

[michaelpj]

We did this