This is done by inheriting most of the code of ConflictFreeLog,
i.e. using a doubly-linked list:
- for fast iteration of the first elements, and in particular
  immediate access to the first element (used for creation date);
- that keeps track of the history length;
- that implement fast reverse iteration (although it could
  have been done without changing the data structure).

The size of buckets is not fixed anymore to 16 items:
like ConflictFreeLog, WorkflowHistoryList is also a good candidate
to look at the estimated serialized size of the bucket in order to
decide if elements should be added to a new one or not.
Then developers won't have to care about using Pdata or not.

The size is bigger than the ConflictFreeLog default,
because workflow items look a lot alike and adding
a few more is cheap when the ZODB compresses.

No more optimized __getstate__ (except for workflow histories that
have not been migrated) so BT export will be a bit more verbose.

The BBB code is because of
  nexedi/erp5!934

/reviewed-on nexedi/erp5!941