multiprocessing now supports shared memory between processes

In the [Python 3.8 release](https://www.python.org/downloads/release/python-380/), multiprocessing now supports [shared memory](https://docs.python.org/3.8/library/multiprocessing.shared_memory.html). I imagine that TPOT could benefit significantly from taking advantage of this shared memory feature when multiprocessing is enabled for a TPOT run.