Interface change! Allow decreasing setOldestVersion

ConflictSet.cpp

@@ -3204,11 +3204,7 @@ Iterator firstGeqLogical(Node *n, const std::span<const uint8_t> key) {
       } else {
         n = nextSibling(n);
         if (n == nullptr) {
-          //  This line is genuinely unreachable from any entry point of the
-          //  final library, since we can't remove a key without introducing a
-          //  key after it, and the only production caller of firstGeq is for
-          //  resuming the setOldestVersion scan.
-          return {nullptr, 1}; // GCOVR_EXCL_LINE
+          return {nullptr, 1};
         }
         goto downLeftSpine;
       }
@@ -3489,8 +3485,8 @@ void checkVersionsGeqOldestExtant(Node *n,
       assert(m >= oldestExtantVersion);
     }
   } break;
-  default:                   // GCOVR_EXCL_LINE
-    __builtin_unreachable(); // GCOVR_EXCL_LINE
+  default:
+    abort();
   }
 }