Make concurrency less brittle

[zml2008]

Currently, due to Gradle restrictions some operations have to be completed on Gradle-owned threads, including any configuration resolution or execution of workers. This leads to some awkward and brittle arrangements to ensure configurations are resolved at the right points, most visible in DecompileJarTask and the infrastructure around produceAssociatedArtifactSync.

Instead, I'd like to see a way to have an Executor that can run on a Gradle thread, using a SynchronousQueue to pass tasks back to the Gradle thread, or something along those lines. This will let VG stay fully in CompletableFuture land for its operations, and more elegantly synchronize only when actually necessary.