Add Python client-based stresstest

[sc1f]

This PR adds a new stresstest module for perspective-python using the new PerspectiveTornadoClient, which is a far superior way to write Perspective stresstests and remote clients in Python. Instead of measuring per-message time, the new stresstest allows the user to write arbitary tasks and measure the progress of those tasks over multiple concurrent clients.

To use the new stresstest, run yarn bench -h to see the available options and configurations. Examples are located in python/perspective/bench/tornado, and can be run as follows:

yarn bench python/perspective/bench/tornado/distributed_mode.py -c10 ws://localhost:8080/websocket

Changelog

• Added new Python client-based stresstest suite
• Removed all code referring to old, custom-written stresstest suite
• Split Python benchmarks into runtime and tornado
• Added new-style benchmarks for distributed mode and server mode

[texodus]

Looks good!

[texodus]

Thanks for the PR!