Fix subtransaction behavior for large objects, temp namespace, files,

Fix subtransaction behavior for large objects, temp namespace, files,
password/group files.  Also allow read-only subtransactions of a read-write
parent, but not vice versa.  These are the reasonably noncontroversial
parts of Alvaro's recent mop-up patch, plus further work on large objects
to minimize use of the TopTransactionResourceOwner.