Run plugins in parallel

Build speed is very important to our users. Should be run plugins in parallel by default?

The following would still happen serially:

• lifecycle hooks, e.g. anything triggered in build always happen before anything in buildFunctions
• when user specifies an array of commands, those would be run in order

However, for a specific lifecycle hook, everything else should be run in parallel.

One downside would be that, by default, plugins would only be able to depend on other plugins that are defined in previous lifecycle hooks. We could introduce a plugin property after to overcome this.

Logs would need to be buffered so that they are displayed to the user as if the plugins had be run serially.

This would provide with a major speed boost.

[DavidWells]

Its looking more and more like we need child processes.

If we use child processes we need to make sure a few things happen:

1. Terminal colors are preserved from the child processes (we had trouble with this in past)
2. Child processes will play nice with plugin outputs
3. Speed/perf doesn’t take huge hit

Child processes might make it easier to scope env vars and other things. So I think its worth checking out

[ehmicky]

I like it! 👍

A follow-up question: logs of parallel plugins should be shown in serial and predictable order (as opposed to interleaved). To do that, they have to be buffered. To be buffered, plugins should be run in child processes.

Running plugins in child processes should be easy, using Node.js stdio: "ipc" to trigger specific hooks. It also has other advantages (see #166 and #167). What do you think?