The DBIx::SearchBuilder cache doesn't help the serializer

The serializer does lots of manual fetching and massaging of collections
(to work around possibly being on old RT/old DBIx::SearchBuilder).
Unfortunately, this means that the ::Cachable record class will fill a
cache that never empties via the normal methods and your rt-serializer
instance will leak memory like a sieve when serializing even 'simple'
objects such as Transactions.

This change made the difference between rt-serializer being unable to
complete on a system with 6G of ram to never using more than 1.5G of
RAM.
Merge branch '4.2/stop-caching-while-serializing' into 4.2-trunk