Optimize WorkflowHistoryList

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 is a bit more verbose.