Change parallel actor graph to be purely push-based. (#70)
In consumer::begin, wait for all threads to initialize.
Remove state_base::done and introduce new worker state DONE.
Handle pushing in end() in parallel.
Merge branch 'issue_70'. Fixes #70.
Move parallel.h to parallel/base.h. (1/2)

Split into two commits to preserve blame information.
Move parallel.h to parallel/base.h. (2/2)

Split into two commits to preserve blame information.
parallel: Split pipes.h off from base.h.
parallel: Split factory.h off from base.h.
parallel: Split options.h off from base.h.
parallel: Split worker_state.h off from base.h.
parallel: Split aligned_array.h off from base.h.
Add parallel.h to CMakeLists.txt.
Merge branch 'parallel_reorg'