I did refactor a few things.

• First used struct slotRangeArray instead of list *slot_ranges. For plugin API, I see that we need to work with primitive types as the current plugin will be in rust, it is harder to use list like complex structs there. struct slotRangeArray is easier to translate to other languages. Also, I see list *slot_ranges does not have much benefit over an array of slot ranges.
• To prevent terminology mixup, I refactored struct slotRange and used start_slot/end_slot instead of first/last.
• Tried to reuse parser code for sflush, cluster import and cluster syncslots