This factors out the concurrency framework in `cmQtAutoGeneratorMocUic` to a
dedicated class `cmWorkerPool` which might be reused in other places.

`cmWorkerPool` supports fence jobs that require that
- all other jobs before in the queue have been processed before the fence
  job processing gets started,
- no jobs later in the queue will be processed before the fence job processing
  has been completed.
Fence jobs are needed where the completion of all previous jobs in the queue
is a requirement for further processing.  E.g. in `cmQtAutoGeneratorMocUic`
the generation of `mocs_compilation.cpp` requires that all previous
source file parse jobs have been completed.