[Dbix-class] Live object index support ported

[Matt S Trout]

I've ported the Class::DBI live object index into DBIx::Class as
::CDBICompat::LiveObjectIndex. Notable changes are -

It's wrapped round insert, _row_to_object and discard_changes, the former
to add new objects, the second to handle retrieved ones and the last to
fiddle around the re-retrieve overwriting the live objects entry.

The integration probably isn't perfect; notably if you use PK::Auto and the
module is before ::LiveObjectIndex in the inheritance chain your new inserts
won't get put in the index because they won't have a pk when ::LiveObjectIndex
sees them go past. I've re-ordered the ISA in Test::SQLite to reflect this.

While porting I made the index and the insert count class data rather than
lexicals - it seems cleaner and I thought it might be interesting to be able
to to localise the object index at any level in the hierarchy, although the
only use I can immediately think of is per-table sweep frequency tuning.

More tests would be appreciated, since the cdbi ones (or at least the ones
imported so far) seem to be a bit light on them. Failing test cases showing
where I've cocked something up would be fantastic :)

-- 
     Matt S Trout           Website: http://www.shadowcatsystems.co.uk
  Technical Director        E-mail:  mst (at) shadowcatsystems.co.uk
Shadowcat Systems Ltd.