pgsql: This patch optimizes the md5_text() function (which is used to

Log Message:
-----------
This patch optimizes the md5_text() function (which is used to
implement the md5() SQL-level function). The old code did the
following:

1. de-toast the datum
2. convert it to a cstring via textout()
3. get the length of the cstring via strlen()

Since we are treating the datum context as a blob of binary data,
the latter two steps are unnecessary. Once the data has been
detoasted, we can just use it as-is, and derive its length from
the varlena metadata.

This patch improves some run-of-the-mill md5() computations by
just under 10% in my limited tests, and passes the regression tests.

I also noticed that md5_text() wasn't checking the return value
of md5_hash(); encountering OOM at precisely the right moment
could result in returning a random md5 hash. This patch corrects
that. A better fix would be to make md5_hash() only return on
success (and/or allocate via palloc()), but since it's used in
the frontend as well I don't see an easy way to do that.

Modified Files:
--------------
pgsql/src/backend/libpq:
md5.c (r1.27 -> r1.28)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/md5.c.diff?r1=1.27&r2=1.28)
pgsql/src/backend/utils/adt:
varlena.c (r1.118 -> r1.119)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/varlena.c.diff?r1=1.118&r2=1.119)