Multiserver events and logging

* Refactor common parts of multiserver worker and and client to a 'base class'
* Add debug messages to multiserver
* Change connect/disconnect event emitting logic
  - `connect` is emitted when changing state from 'all servers disconnected' to 'single server connected'
  - `disconnect` is emitted in the opposite case
* Change gearslothd on connect info message
* Enable require('./lib/gearman').Worker pattern