flat_hash_map is recommended over node_hash_map by absl. This avoids a pointer indirection when accessing keys.

When the hash table has more than 50% utilization (the common case, given resize is triggered at 87.5% utilization), we use less memory (solve for utilFraction in 8*capacity + 16*utilFraction = 16*capacity).

checkMemoryUsageAndSpillIfNecessary already does not account for the 8 byte MaterializedRow pointer in _ht, so we don't need to change it here.