AutoScaler: calls listener if max size reached

- tracks the "unbounded pool size" (i.e. what it would be if there were
  no max pool size), and calls a listener if this goes above the
  max pool size for a sustained period of time
AutoScaler tidy: extract SizeHistory class
Fix REST's ConfigSummary.defaultVal serialization

- When config key's default value is not json serialisable, was getting
  500 errors.
- Fix by just sending String representation of the default value
AutoScalerPolicy: support reconfiguring config
AutoScalerPolicy: emit maxSizeReached sensor

- Instead of calling listener, emit a sensor event on the associated
  entity
AutoScalerPolicy maxSizeReached: incoroporate review comments