[fix](vec) Fix some bugs about vec engine (#7884)

1. mem leak in vcollector iter
2. query slow in agg table limit 10
3. query slow in SSB q4,q5,q6

be/src/vec/columns/column_string.cpp

@@ -323,6 +323,7 @@ void ColumnString::replicate(const uint32_t* counts, size_t target_size, IColumn
 
 void ColumnString::reserve(size_t n) {
     offsets.reserve(n);
+    chars.reserve(n);
 }
 
 void ColumnString::resize(size_t n) {