Updates based on 3nd round of feedback and discussions

- add experimentalSettings to AkkaBuild.scala
- allow processor recovery to fail
- rewrite Processor to make states and transitions more explicit
- do not use Enumeration
- underscore for _channelId constructor parameter
- allow implementors to opt out from restarting children
- rename Message to Persistent
- send persisted message to processor after being written to journal
 - use implicit sender for Write and Loop commands