Replace Settings::reconverge with Option<u32> to indicate the ID of reconverge for the applicable node

[minshao]

After the removal #273, the only field related to REconverge in the Settings would be a boolean flag indicate whether the node contains reconverge.

• In the future, it would be more informative to keep something similar to Option<u32>, where the u32 indicates the id of reconverge.
• Reconverge id would be available once we completed setting up profile storage for REconverge instance.

Blocked: currently, REconverge profile is not stored in review-database and, thus, REconverge instance doesn't have an ID yet.