Stop caching when serializing and importing

9b26694d and ba372275 rightly realized that using
DBIx::SearchBuilder::Record::Cachable was large memory and performance
detriment when serializing and importing.  Unfortunately, their changes
were unsufficient to prevent it, as RT::Record is loaded (and its "use
base" is set) during compile-time when
RT::Migrate::Serializer::IncrementalRecord is loaded.  As such, setting
RecordBaseClass at run-time is too late.

Instead, explicitly alter the inheritance of RT::Record at run-time, to
avoid the complexity of ensuring that RT::Record is not loaded until
"late enough."
Skip building a giant "seen" hash if cloning

There is no reason to build the "seen" hash if we simply ignore it;
instead, simply skip directly to the Visit code.
Merge branch '4.2/serializer-leaks' into 4.2-trunk