pgsql-server: Revise ResourceOwner code to avoid accumulating

Log Message:
-----------
Revise ResourceOwner code to avoid accumulating ResourceOwner objects
for every command executed within a transaction. For long transactions
this was a significant memory leak. Instead, we can delete a portal's
or subtransaction's ResourceOwner immediately, if we physically transfer
the information about its locks up to the parent owner. This does not
fully solve the leak problem; we need to do something about counting
multiple acquisitions of the same lock in order to fix it. But it's a
necessary step along the way.

Modified Files:
--------------
pgsql-server/src/backend/access/transam:
xact.c (r1.178 -> r1.179)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/xact.c.diff?r1=1.178&r2=1.179)
pgsql-server/src/backend/utils/mmgr:
portalmem.c (r1.68 -> r1.69)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/utils/mmgr/portalmem.c.diff?r1=1.68&r2=1.69)
pgsql-server/src/backend/utils/resowner:
README (r1.2 -> r1.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/utils/resowner/README.diff?r1=1.2&r2=1.3)
resowner.c (r1.2 -> r1.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/utils/resowner/resowner.c.diff?r1=1.2&r2=1.3)