Investigate communicating worker completion over IPC, rather than exit signals

[novemberborn]

For #1722 I had to remove the job concurrency in AVA's internal tests for Windows. Tests that themselves invoked AVA in a child process would time out, since the invoked AVA instance would fail to detect its workers exited.

There's something odd going on with detecting process exits on Windows. This seemed more prevalent with Node.js 6. I'm not sure if this is a known issue.

Regardless, we should investigate whether the worker can communicate its "I'm about to exit successfully" over IPC, to increase the likelihood of AVA detecting this correctly.

See also #1718 and #1378.

[novemberborn]

I've seen timeout behavior still on Windows, so old though this issue may be, it could still be a worthwhile change.