[SPARK-2764] Simplify daemon.py process structure

Curently, daemon.py forks a pool of numProcessors subprocesses, and those processes fork themselves again to create the actual Python worker processes that handle data.

I think that this extra layer of indirection is unnecessary and adds a lot of complexity.  This commit attempts to remove this middle layer of subprocesses by launching the workers directly from daemon.py.

See https://github.com/mesos/spark/pull/563 for the original PR that added daemon.py, where I raise some issues with the current design.
In most cases, EAGAIN should be recovable, should we also catch that?