Fix locking problems during shutdown

[msimberg]

This is to test if this helps for the Windows shutdown problems, but I will still see if we can get by with the stopped_ and terminated_ flags being atomic<bool>s instead (the mutex has to be locked in any case for the condition variables).

Proposed Changes

• Unlock mutex while yielding/suspending HPX thread during shutdown
• Includes Resolving problems during shutdown for VS2015 #3203

Any background context you want to provide?

HPX threads can migrate between OS threads. Holding an OS mutex in an HPX thread which gets migrated to another OS thread is undefined behaviour (this is checked on Windows). For #3203 a mutex was being held while being destructed.

[hkaiser]

This might actually make #3203 obsolete.

[hkaiser]

LGTM, thanks!