- Added missing includes.
- Added missing namespace qualification for Boost.Chrono casts.
- Added typedefs renaming polymorphic lockable wrappers to a more consistent
  naming scheme. This fixes incorrect name references.
- Fixed incorrect duration casts in polymorphic lockable wrappers.
- Renamed upgrade_lockable_adapter to poly_upgrade_lockable_adapter, since
  upgrade_lockable_adapter already exists and has a different meaning.
- Use BOOST_THREAD_FUTURE instead of future in executors and task region.
- Disable serial_executor_cont when executors or future continuations
  are not enabled.
- Marked sync_queue_is_closed exception as visible.