Add awareness of init status while creating listeners

* Slave.Source: Don't fulfill `listen` requests while we're in the middle
  of loading a rewind buffer. Queue requests and load them after the buffer
  is in place.
* Outputs: `stream.listen` now takes a callback, and can now take either
  `offsetSecs` or `offset` (chunks)
* RewindBuffer: Make it more explicit what functions take seconds, and what
  functions take a chunk index
Rewind: Rework pump functions to better support pumper output

* Rewind: pump functions now take a Rewinder and call `_insert`, rather than
  returning a single chunk.  Allows better integration with the rest of the
  Readable pipeline
* Rewinder: Outputs looking for pumpOnly should listen for the `pump` event,
  which contains length, duration, etc.
Rewind: Change burst funcs to use callbacks

* Update `burstFrom` to support the new `pumpFrom` call style