Callback naming consistency

Following up on @vitaly-krugl’s comment, we’re going to use the on_ prefix with an action verb for all callback parameter names.

[vitaly-krugl]

@lukebakken, regarding documentation comment in https://github.com/pika/pika/issues/988#issuecomment-370978795, I was hoping to get the apps away from accessing the connection’s internal ioloop, which is often an adapter on top of the native I/O loop. I would not want them to depend on any connection.ioloop API (that’s should really be just for the benefit of adapter implementations), calling it’s start method, etc. (it shouldn’t be even providing a start() method). Instead, apps should be interacting with the native I/O loop directly - calling the native loop’s start(), run_forever(), etc.

I started going on this path in my WIP PR https://github.com/pika/pika/pull/982.

[vitaly-krugl]

Making the callback names more descriptive would help with making the API more self-documenting. But things are not bad now, so I’m not certain it’s worth the extra effort. I’m okay to keep as they are.