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).

[ci skip]
Initialize read_timeout to avoid 'warning: instance variable @read_timeout not initialized'.
Merge pull request #399 from simi/benchmarks_update

Update benchmarks.
Merge pull request #400 from simi/avoid_warning

Initialize read_timeout
make sure write_timeout and local_infile ivars are setup to avoid warnings
get rid of some more warnings
clean up connections in specs where we can
some more spec cleanup