Speed Improvements to Stage Execution Times

[Justintime50]

• The Build stage is INCREDIBLY fast - wahoo!
• The Test stage needs help. Because we are building unique images every single time with unique tags and unique containers - nothing can get cached. This is great for security and containing each persons tests to a unique container but is terrible on performance. Some scripts that only take 1-3 seconds to run are having their test stage take 15-20 seconds as all the overhead is on Docker.
• The Deploy Stage is fine, spinning up the container isn't the problem - it comes with tearing down the old one. Many containers will wait the default 10 seconds before stopping pending a "graceful shutdown" but many of the projects configured don't have a graceful shutdown in place and therefore simply wait the 10 default seconds. Killing containers immediately may not be a great idea because some projects will have a graceful shutdown - we need to find a happy median where we can shut down containers ASAP. This may require a per-project config.

[Justintime50]

Speed improvements would also come if we had a workflow like Travis where Travis provided a pre-built container with most things already cached and downloading cached libraries only as needed inside the container - this however leaves out some flexibility that Harvey currently allows. Additionally, it would take a lot of effort to get a comprehensive container put together that spans various programming languages and versions with shells, paths, etc all ready to go.

[Justintime50]

This may have been helped slightly by switching from threads to processes as of #17

[Justintime50]

Switched back from processes to threads as it crashed Docker on older machines. No noticeable speed improvements on machines capable either (may have improvements on multiple concurrent builds, but a single build didn't see any improvements.)