Add a record iteration protocol to FileStorage.  You can use the record iterator to iterate over all current revisions
of data pickles in the storage.

In order to support calling via ZEO, we don't implement this as an actual iterator.  An example of using the record iterator
protocol is as follows:

storage = FileStorage('anexisting.fs')
next_oid = None
while 1:
    oid, tid, data, next_oid = storage.record_iternext(next_oid)
    # do something with oid, tid and data
    if next_oid is None:
        break

The behavior of the iteration protocol is now to iterate over all current records in the database in ascending oid order, 
although this is not a promise to do so in the future.