Conversation handlers

[PaulSonOfLars]

What

Implement a new type of Handler, the conversation handler, which allows for storing the state of each user and emulate a "conversation" with them.

This does not add support for conversation timeouts or forced linear processing of conversation events; those can be added in the future, when needed.

Impact

• Are your changes backwards compatible? No; change to the handler interface signature.
• Have you included documentation, or updated existing documentation? Y
• Do errors and log messages provide enough context? Y