to create multiple IFile in a batch.

For example during clean-build JDT first deletes all output folders and
then writes one .class file after the other. Typically many files are
written sequentially. However they could be written in parallel if there
would be an API.

This change keeps all changes to the workspace single threaded but
forwards the IO of creating multiple files to multiple threads.

The single most important use case would be JDT's
AbstractImageBuilder.writeClassFileContents()

The speedup on windows is ~ number of cores, when they have hyper
threading.

OutOfMemory is not to be feared as the caller has full control how many
bytes he passes.