Reorganize code for operation modes into separate files
Fix Master and Slave modes

* Move HandoffTranslator to modes/base
* Master: Be consistant in emitting 'config', not 'streams'
* Master: Add `stream_stats` call for slaves, to get stream vitals
* Master.Stream: Implement `_vitals()`
* Master Mode: Fix admin server setup and start to prepare for handoffs
* Slave Mode: Start to prepare for handoffs
* RewindBuffer: Add a listener to allow sources to emit `vitals` instead
  of header.
* Slave: Ask for vitals during SocketSource setup