Remove automatic call to next() in deleteMessage() and changeMessageVisibility()

When running with a high concurrency level and a queue with many message it is highly probable that the call to deleteMessage() will get scheduled for execution longer in the future than the visibility timeout of the received message.

This pull request removes the automatic call to next() and instead expects the user to call it explicitly.  For safety it can be called in the deleteMessage() callback.  For speed it can be called immediately after calling deleteMessage().

Also updated the docs and the tests.