Effect of Node48 NEON search unrolled x2

baseline, patch

Speedups (negative means slowdowns):

• n48_sequential_add<unodb::db>/2: 4%
• n48_sequential_add<unodb::db>/8: 5%
• n48_sequential_add<unodb::db>/64: 4%
• n48_sequential_add<unodb::db>/512: 2%
• n48_sequential_add<unodb::db>/4096: 2%
• n48_random_add<unodb::db>/2: 5%
• n48_random_add<unodb::db>/8: 6%
• n48_random_add<unodb::db>/64: 7%
• n48_random_add<unodb::db>/512: 3%
• n48_random_add<unodb::db>/4096: 5%