remove identical item during index merge

[caterchong]

Is your feature request related to a problem? Please describe

Normally, there is no identical index item in merge set. So mergeset/merge.go don't consider this scenorio in (bsm *blockStreamMerger) Merge.

But it still make sense for some offline data processing. for example, merging serveral backups into one.
Removing identical items help to shrink the size of index.

Describe the solution you'd like

if current item equals next item ,just skip it.
the code might look like this:

for bsr.currItemIdx < len(items) {
		item := items[bsr.currItemIdx].Bytes(data)
		itemStr := string(item)
		if compareEveryItem && itemStr > nextItem {
			break
		}

		// if equal to nextItem, skip this item
		if itemStr != nextItem {
			if !bsm.ib.Add(item) {
				// The bsm.ib is full. Flush it to bsw and continue.
				bsm.flushIB(bsw, ph, itemsMerged)
				continue
			}
		}

		bsr.currItemIdx++
	}

this change requires a string == calculation, and performance call be improved by optimize string(item).

Describe alternatives you've considered

No response

Additional information

No response