dynamic history

Previously, all specs had to be pre-allocated by using the 'add_history'
function. This is now no longer required and instead serves as a hint to
the HistoryContainer to pre-allocate the space for the given spec.

History can grow by increasing the length for a frequency, adding a
frequency, or adding a field. It can grow with any combination of
these.
Am I correct in reading that `window_starts` and `window_closes` are meaningless if `_is_buffer` is True?  Seems like this should be part of a different function if that's the case.