Effect of Node48 AVX2 insert position search unrolling x4

baseline commit, patch

Speedups (negative means slowdowns):

• n48_sequential_add<unodb::db>/2: 5%
• n48_sequential_add<unodb::db>/8: 4%
• n48_sequential_add<unodb::db>/64: 5%
• n48_sequential_add<unodb::db>/512: 2%
• n48_sequential_add<unodb::db>/4096: 1%
• n48_random_add<unodb::db>/2: 2%
• n48_random_add<unodb::db>/8: 10%
• n48_random_add<unodb::db>/64: 6%
• n48_random_add<unodb::db>/512: 4%
• n48_random_add<unodb::db>/4096: 2%