Upload the binaries and files of a C++ component in parallel

Publishing of C++ libraries and applications offer plenty of opportunities to perform work in parallel:

- Each library and application can be uploaded in parallel, as can each variant of a component. Change the upload tasks to use the worker API and so run in parallel with each other.
- Each variant includes several files. Upload each of these in parallel.

The implementation should also take care of coordinating the upload of the pom and module metadata file so that components become visible in the repository in dependency-first order, and that a component becomes visible only after all of its variants are visible.

This isn't a C++ specific feature, but the nature of C++ components means that there are more files to upload than, say, a JVM library.

Start by locating the upload tasks for C++ libraries and applications and the worker API they use. The work is done when libraries, applications, variants, and variant files upload in parallel while POM and module metadata preserve dependency-first visibility and components appear only after all variants are visible.

Written by the indexing model from the issue text.