Improve fork-safety

[rohanpm]

I haven't investigated the behavior on fork, but as several futures created by this library are backed by threads and as threads aren't copied over a fork, it seems likely that an application using more-executors and fork() could end up with a bunch of futures which hang when accessed.

It would be nice if this could be improved. It can't be made to "work", but it could possibly be made to fail in a predictable way.

For example, can we make it so that after a fork, any future which depended on a disappeared thread is automatically failed?

[rohanpm]

Executors themselves would also have to be marked either as dead, or have the ability to restart their dead worker threads.