Revert support for isolates.

It was decided that the performance benefits that isolates offer (faster spin-up
times for worker processes, faster inter-worker communication, possibly a lower
memory footprint) are not actual bottlenecks for most people and do not outweigh
the potential stability issues and intrusive changes to the code base that
first-class support for isolates requires.

Hence, this commit backs out all isolates-related changes.

Good bye, isolates. We hardly knew ye.