Breaking changes for v2.0

[llucax]

This is a tracking issue to document all breaking changes we want to introduce for v2.0.

Once we have a more clear plan to go for 2.0, we can convert some of the TODOs in their own issues to make tracking of different changes easier.

• Make Receiver.close() abstract.

• In new_receiver(), rename the argument limit to buffer_size.

• Rename 1.x Timer to MonotonicTimer. Add a Timer that works with the wall clock.

• Add a tick_on_init parameter to Timer and change tick behaviour to the following:

  tick_on_init	start_delay	tick at 0	tick after delay
  true	> 0	true	true
  true	0	true	-
  false	> 0	false	true
  false	0	false	-

• Remove channel.close() method, which has been replaced by channel.aclose()

[llucax]

When we have a wall clock timer, I feel the complexity of the monotonic timer can be greatly reduced, as many of the features needed there are just to support using it as a base to construct wall clock timers, so I hope in v2 we can actually remove complexity from the monotonic timer (like start_delay) instead of adding more.