Effect of initializing Node48 child_indexes at declaration

baseline commit, patch

• micro_benchmark_n48 --benchmark_filter='(grow|shrink).*unodb::db': 4% slowdown (shrink_n256_to_n48_randomly<unodb::db>/512) to 3% speedup (shrink_n256_to_n48_sequentially<unodb::db>/2048)
• micro_benchmark_n48 --benchmark_filter='(grow|shrink).*unodb::olc_db': 0% (shrink_n256_to_n48_sequentially<unodb::olc_db>/4) to 26% speedup (grow_n16_to_n48_sequentially<unodb::olc_db>/512)