Revise memory management for libxml calls. Instead of keeping libxml's data

in whichever context happens to be current during a call of an xml.c function,
use a dedicated context that will not go away until we explicitly delete it
(which we do at transaction end or subtransaction abort).  This makes recovery
after an error much simpler --- we don't have to individually delete the data
structures created by libxml.  Also, we need to initialize and cleanup libxml
only once per transaction (if there's no error) instead of once per function
call, so it should be a bit faster.  We'll need to keep an eye out for
intra-transaction memory leaks, though.  Alvaro and Tom.