Support Oracle 12c top-N query

[yahonda]

This pull request has two separate commits to support Oracle 12c better Top N query support using FETCH FIRST n ROWS ONLY and OFFSET n ROWS.

8f5e57f Extract visit_Arel_Nodes_SelectOptions

It extracts visit_Arel_Nodes_SelectOptions from visit_Arel_Nodes_SelectStatement method to prepare Oracle 12c support since it will be able to reuse most of super's visit_Arel_Nodes_SelectStatement method because Oracle requires to add collector in the following order.

offset
limit
lock

Unlike other database require in this order.

limit
offset
lock

This method is not called using visitor pattern, I'd like to hear visit_Arel_Nodes_SelectOptions is an appropreate method name.

371b642 Create Arel::Visitors::Oracle12 to provide better top-N query

Oracle Database 12c introduces better top-N query support using FETCH FIRST n ROWS ONLY and OFFSET n ROWS. Also to support older version of Oracle 11gR2 or lower version, this commit creates
another visitor named Arel::Visitors::Oracle12, 12 comes from the database version.

By changing oracle enhanced adapter as follows to execute each Arel visitor based on
if this top-n query feature supported. yahonda/oracle-enhanced@fcb22db

if supports_fetch_first_n_rows_and_offset?
  @visitor = Arel::Visitors::Oracle12.new self
else
  @visitor = Arel::Visitors::Oracle.new self
end

It looks Arel::Visitors just allows 1 database adapter to have 1 visitor, but my implementation works to switch two visitors just changing Arel::Visitors::Oracle12 or Arel::Visitors::Oracle based on its database version. It looks 1 database adapter can have 2 or more visitor(s). Are these lines can be removed?
https://github.com/yahonda/arel/blob/a19190ab06db4d68765fb08ec1a4dc6c8ebb2eb5/lib/arel/visitors.rb#L15-L41

module Arel
  module Visitors
    VISITORS = {
      'postgresql'      => Arel::Visitors::PostgreSQL,
      'mysql'           => Arel::Visitors::MySQL,
      'mysql2'          => Arel::Visitors::MySQL,
      'mssql'           => Arel::Visitors::MSSQL,
      'sqlserver'       => Arel::Visitors::MSSQL,
      'oracle_enhanced' => Arel::Visitors::Oracle,
      'sqlite'          => Arel::Visitors::SQLite,
      'sqlite3'         => Arel::Visitors::SQLite,
      'ibm_db'          => Arel::Visitors::IBM_DB,
      'informix'        => Arel::Visitors::Informix,
    }

    ENGINE_VISITORS = Hash.new do |hash, engine|
      pool         = engine.connection_pool
      adapter      = pool.spec.config[:adapter]
      hash[engine] = (VISITORS[adapter] || Visitors::ToSql).new(engine)
    end

    def self.visitor_for engine
      ENGINE_VISITORS[engine]
    end
    class << self; alias :for :visitor_for; end
  end
end

[yahonda]

Failed to merge master branch. will close this pull request.