Documentation and channel enhancements

Do not use channels as primary confirm destinations

- they may become invalid if their parent processor is restarted
- many successful confirmations can get lost this way
- instead, use the journal actor as primary confirm destination
- include the channel reference into the confirm message
- let the journal forward the confirmation to the channel (after writing the confirmation)
- if the channel doesn't exist any more, it's ok if the confirmation goes to deadLetters

Documentation enhancements

- channel usage warning
- reliable event delivery
- how to use LevelDB with sbt

Tests

- remove obsolete tests
- add further redelivery tests
- add Channel to FailureSpec