Notifier stub.

Process::name() implementation.
ProcessGroup::notifier() implementation.
notifier::ProcessId declaration.
notifier::SpawnEvent declaration.
notifier::TerminationEvent declaration.
Notifier::Event declaration.
notifier::BlockStream implementation.
notifier::ObjectStream implementation.
Notifier implementation.
Notifier refactoring.
detail::execution::async_process_group_detail::ProcessGroupStarter refactoring.
notifier::QueuedWriter implementation.
notifier::ProcessId -> ProcessMeta.
ProcessMeta integration.
detail::execution::async_process_group_detail::ExecutionMonitor refactoring.
detail::execution::async_process_group_detail::ProcessGroupStarter style fix.
ProcessMeta: operator<<() implementation.
detail::execution::async_process_group_detail refactoring.
CMakeLists.txt refactoring.
Notifier integration.
ContainerConfig refactoring.
AsyncProcessGroupFixture fix.