`atomic_fork_group` - to fork/join from different threads

[tzcnt]

Currently fork_group has non-atomic counters. This is optimal for performance when fork and join are from the same task, but it's not safe to fork on multiple tasks/threads simultaneously, or fork on a different task from the join task without synchronization, if those tasks might run on different threads.

A fully atomic version should support these use-cases:

• Task trees where inner tasks create forked tasks that the root can await.
• A safer version of detach() / post() that allows creating background tasks at various times in the application, and waiting for them to complete before shutdown.

Maybe support:

• Awaiting fork_group from multiple threads simultaneously / awaiting multiple times. Each await should yield the same value. This would change fork_group into an lvalue awaitable. This may relate to result_share() / result_ref() awaitable customizations #62