mtr_t::mtr_t() allocates some memory

This was a rather surprising find while profiling.
mtr_t::mtr_t allocates memory through construction of one of its members

This allocations accounts for 0.68% of CPU time (constructor itself for 0.77% of CPU time), this happens to be the largest contributor to the "operator new" on oltp_update_index benchmark with bufferpool larger than size of the data ( without doublewrite)