refactor(core): context stage 2

[Archento]

List of changes:

• includes (and depends on) changes from refactor(core): context stage 1 #349
• add communication module for handling sending of messages
• add Dispenser class in communication module
• add new background task in agent.py for message dispensing
• add new Context interface / Abstract Base Class to be used by the handlers
• split existing Context into two types:
  • InternalContext: for supporting pro-active behaviour (on_event, on_interval)
  • ExternalContext: for supporting re-active behaviour (on_message)
• drop protocol discovery feature
• remove duplicate code
• remove background_tasks set

[Archento]

Sync messages and queries still need to be done but it works for most of the examples and should be ready for initial feedback.

[jrriehl]

A few remaining subtleties to address, but overall this looks great.

The raised issues are valid and will be revisited in the next iteration / a separate issue.

[jrriehl]

LGTM