Options is needed for some Rails extensions to determine when
referential integrity should be disabled

This reverts commit bcb466c5.

Fixes #5052

This allows the internal :_dump task to be called as many times as
required. For instance, `rake db:migrate:redo` calls two tasks that
internally call :_dump, and without reenabling it to be reinvoked,
the database was left in incorrect state.

behavior.

would get ConnectionNotEstablished error because it always tried to use
ActiveRecord::Base's connection, even though it should be using the connection
of the model whose context we're operating in

The `add_index` method now supports a `where` option that receives a
string with the partial index criteria.

    add_index(:accounts, :code, :where => "active")

    Generates

    CREATE INDEX index_accounts_on_code ON accounts(code) WHERE active