Use different name for main and thread connection variable.

Under JRuby, the updates of the one shared variable interleaved,
causing threads to pick up each others' connections. I'm amazed
this worked on MRI.

activerecord/test/cases/connection_pool_test.rb

@@ -184,14 +184,14 @@ def test_active_connection?
 
       def test_checkout_behaviour
         pool = ConnectionPool.new ActiveRecord::Base.connection_pool.spec
-        connection = pool.connection
-        assert_not_nil connection
+        main_connection = pool.connection
+        assert_not_nil main_connection
         threads = []
         4.times do |i|
           threads << Thread.new(i) do
-            connection = pool.connection
-            assert_not_nil connection
-            connection.close
+            thread_connection = pool.connection
+            assert_not_nil thread_connection
+            thread_connection.close
           end
         end