storage: restore lower-bound of 1 on pebbleMVCCScanner.itersBeforeSeek

This was supposed to be temporarily removed in
b258e82
but was never restored.
It accounts for a 2x slowdown in a benchmark where a scan encounters a
few keys at the beginning of the scan with > 5 versions, that cause
itersBeforeSeek to drop to 0, and then for the remaining 1000s of keys
with only 1 versions it uses SeekGE instead of Next.

Informs #96361

Also see cockroachlabs/support#2033

Epic: none

Release note: None

pkg/storage/pebble_mvcc_scanner.go

@@ -551,8 +551,13 @@ func (p *pebbleMVCCScanner) incrementItersBeforeSeek() {
 // Decrements itersBeforeSeek while ensuring it stays positive.
 func (p *pebbleMVCCScanner) decrementItersBeforeSeek() {
 	p.itersBeforeSeek--
-	if p.itersBeforeSeek < 0 {
-		p.itersBeforeSeek = 0
+	if p.itersBeforeSeek < 1 {
+		if maxItersBeforeSeek > 0 {
+			p.itersBeforeSeek = 1
+		} else if p.itersBeforeSeek < 0 {
+			// INVARIANT: maxItersBeforeSeek == 0 && p.itersBeforeSeek < 0.
+			p.itersBeforeSeek = 0
+		}
 	}
 }