process: add ProcessDriver to handle orphan reaping

[ipetkov]

Motivation

• Dropping/canceling child processes cannot be done cleanly since we can neither afford to perform a blocking wait call in the destructor, nor do we have something like async-destructors to perform the clean up for us
• Although the kill_on_drop flag will terminate the child process on drop, there is no guarantee that the caller will call child.await which will leave a zombie process floating around and can lead to exhaustion of the pid space
• We do try to clean up orphaned processes on a best-effort-basis, as long as additional child processes are being spawned/polled, but this isn't good enough since enough zombie processes can accumulate through one way or another

Solution

• Introduce a ProcessDriver (only on Unix platforms) which will reap the orphaned processes spawned by tokio, gradually eliminating them as they finally exit.
• This also avoids having every child process wrapper attempt to drain the orphan queue, which should lead to faster poll times and fewer contentions for the orphan queue lock.

Refs #2685
Refs #2841

[carllerche]

This does not have to happen in this PR. I believe that, instead of defining an Either for each individual component (I/O, signal, process), you probably can have a single Either for the entire "I/O driver stack". This is because, at runtime, you can either enable I/O, signal, and process or disable all of them.

[ipetkov]

Good idea, this was all left over from before we decided to group the I/O stack together. I'll have to think a bit about how to implement this and I'll open a follow up PR

[carllerche]

LGTM. Added a thought for cleanup that can be done in a separate PR.