Make ProxyChain thread-safe

[saidie]

This PR fixes the issue #5 by using thread local variable in ProxyChain.
Sorry in advance if the name of the thread local variable is not preferable.

[saidie]

I have a test case which fails without this patch but I'm hesitating to include it since it uses ugly sleep:

require "spec_helper"

describe "ProxyChain thread-safety" do

  class Proxy < Arproxy::Base
    def execute(sql, name)
      sleep 0.01
      { connection: self.proxy_chain.connection }
    end
  end

  module ::ActiveRecord
    module ConnectionAdapters
      class DummyAdapter
        def execute(sql, name = nil)
        end
      end
    end
  end

  let(:connection) { ::ActiveRecord::ConnectionAdapters::DummyAdapter.new }

  before do
    allow(Arproxy).to receive(:logger) { Logger.new('/dev/null') }
    Arproxy.clear_configuration
    Arproxy.configure do |config|
      config.adapter = "dummy"
      config.use Proxy
    end
    Arproxy.enable!
  end

  after(:each) do
    Arproxy.disable!
  end

  context "with two threads" do
    let!(:thr1) { Thread.new { connection.dup.execute '', '' } }
    let!(:thr2) { Thread.new { connection.dup.execute '', '' } }

    it { expect(thr1.value[:connection]).not_to eq(thr2.value[:connection]) }
  end

end

[mirakui]

Thank you for this PR! Released as v0.2.1.