Prefer enum class over unscoped enums

[msimberg]

We should gradually move all unscoped enums to enum classes.

Some of those listed below could possibly be hidden in some detail namespace...

• [x] thread_state_enum
• [x] thread_state_ex_enum
• [x] thread_stacksize
• [x] thread_priority
• [x] scheduler_mode (#5809)
• [x] scheduler_state (#5809)
• [ ] component_enum_type
• [x] id_type::management_type (#5817)
• [x] cache::statistics::method (#5816)
• [ ] commandline_error_mode
• [ ] hpx::error
• [x] hpx::throwmode (#5820)
• [x] future_state
• [x] performance_counters::counter_type (#5850)
• [ ] performance_counters::counter_status
• [ ] performance_counters::discover_counter_mode
• [ ] agas::service_mode
• [x] serialization::archive_flags
• [x] serialization::chunk_type
• [ ] serialization::exception_type
• [ ] hpx_hwloc_membind_policy
• [ ] message_buffer_append_state (parcel coalescing)
• [ ] distribution_type (affinity)
• [ ] deque_status_type (boost lockfree deque)
• [ ] context_state, context_exit_state, context_exit_status (coroutines)
• [ ] logging_destination (logging)
• [ ] partitioner_mode

… there are more, please add to this list, if needed.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[gonidelis]

@hkaiser checked throwmode