Factor out ScriptChildProcess class

[aomarks]

Factor out a new ScriptChildProcess class from the#executeCommandIfNeeded method. This class represents a spawned child process.

This will be useful for server mode, because it gives us a more discrete object that represents a running child process. The lifecycle of running processes will become more complex in server mode, especially in watch mode where we'll be persisting child processes potentially across different build graphs. In a follow up PR, it will gain a state property and a terminate method.

Also includes a small tweak to logging. We used to log the "Running" message before we tried to get a WorkerPool slot, which could give the impression that something was running that wasn't yet, when concurrency was constrained.

[aomarks]

Sorry, just force pushed because I realized the incremental diff was actually more confusing than a full diff in this case. Ignore previous comment about looking at these one-by-one.