Add config to disable default node hash index

[adsharma]

Related: #97

  CALL enable_default_hash_index=false;
  CREATE NODE TABLE `User` (`name` STRING,`age` INT64, PRIMARY KEY(`name`));
  UNWIND range(1, 10000) AS i
  CREATE (:User {name: CAST(i AS STRING), age: i});

and verified to contain 10000 rows.

The persisted on-disk result is now clearly different:

• test10k.db: 3.1M
• test10k-nohash.db: 116K

CALL disk_size_info() after reopening shows:

• test10k-nohash.db: only header, catalog, metadata, node_table, total=29 pages
• test10k.db: also has pk_index_overhead and index_data, total=801 pages