Update benchmarks.

- Upgraded to new ActiveRecord API (table_name, limit, to_a).
- Locked to ActiveRecord >= 3.0. ^
- Used Mysql2::Error instead of undefined Mysql2::Mysql2Error.
- Specify root user to sequel benchmark. Other benchmarks are using password less
  root account.
- Fixed picking random array value in benchmark/setup_db.rb (1.8 and 1.9
  compat).