Managed state

[Fishrock123]

Essentially an extension of https://github.com/Fishrock123/bob/issues/53 with a different focus.

There are a number of reasons (#53, #52, #30, etc) for why some kind of state management that does not need to be reimplemented by every stream would be useful.

There are two primary ways to deal with this (that I can think of):

• Class inheritance, do everything in a base class and then extend that
• Some kind of managed state object which is persistent and passed in the pull flow
  • @jasnell's idea
  • Could potentially deal with the issues without requiring class inheritance?

Of course, if we inherit form a class the obvious thing to do would be to integrate the verify transform into said class, so that guarentees are at the absolute minimal by convention but rather by code.