Added "register_death_callback()" interface to run code at the begining of

Added "register_death_callback()" interface to run code at the begining of
"kill_me()".  This is the first thing needed to provide a transparent transaction
interface somewhere down the line.  It is added now to support database transactions
that span multiple requests.  We need this in order to send a ROLLBACK to the DB
in the case of a session killing error during a DB transaction.  We don't want
to leave locks or failed transactions lying around in the DB, right?

git-svn-id: svn://svn.open-ils.org/OpenSRF/trunk@94 9efc2488-bf62-4759-914b-345cdb29e865