Fix mutex deadlock in setup_connection (#128)

Adds auto_release = false in setup_connection to avoid trying to release
the connection to the pool before it has been added.

spec/database_spec.cr

@@ -8,6 +8,7 @@ describe DB::Database do
 
       db.setup_connection do |cnn|
         cnn_setup += 1
+        cnn.scalar("1").should eq "1"
       end
 
       cnn_setup.should eq(2)