row_cache: partition_snapshot_row_cursor: Print more details about the current version vector

Now the format is the same as for the "heap" version vector. Contains
positions and continuity flags. Helps in debugging.

Before:

  {cursor: position={position: clustered,ckp{...},-1}, cont=0, rev=1, current=[0], heap=[
  ], latest_iterator=[{position: clustered,ckp{...},-1}]}

After:

 {cursor: position={position: clustered,ckp{...},-1}, cont=0, rev=1, current=[{v=0, pos={position: clustered,ckp{...},-1}, cont=false}], heap=[
  ], latest_iterator=[{position: clustered,ckp{...},-1}]}

partition_snapshot_row_cursor.hh

@@ -659,7 +659,7 @@ public:
                 out << ", ";
             }
             first = false;
-            out << v.version_no;
+            out << "{v=" << v.version_no << ", pos=" << v.it->position() << ", cont=" << v.continuous << "}";
         }
         out << "], heap=[\n  ";
         first = true;