add :delay, :rolling-min, and :rolling-max

[brharrington]

Add operators requested for alerting use-cases:

• delay: can be used as an alternative to :offset for
  use-cases short offsets that need to run in the streaming
  path. Also it can be applied to any time series instead of
  just data expressions.
• rolling-min: track the minimum value seen within a
  window. For data that is mostly smooth, but has some noise
  this can be a useful way to get a reasonable lower bound
  with little tuning required.
• rolling-max: similar to rolling-min only using the max
  to get an upper bound.

Also refactors DES operators to use a new common base trait
that works for any implementation of OnlineAlgorithm.