pgsql: When using the OSSP UUID library, cache its uuid_t state object.

When using the OSSP UUID library, cache its uuid_t state object.

The original coding in contrib/uuid-ossp created and destroyed a uuid_t
object (or, in some cases, even two of them) each time it was called.
This is not the intended usage: you're supposed to keep the uuid_t object
around so that the library can cache its state across uses. (Other UUID
libraries seem to keep equivalent state behind-the-scenes in static
variables, but OSSP chose differently.) Aside from being quite inefficient,
creating a new uuid_t loses knowledge of the previously generated UUID,
which in theory could result in duplicate V1-style UUIDs being created
on sufficiently fast machines.

On at least some platforms, creating a new uuid_t also draws some entropy
from /dev/urandom, leaving less for the rest of the system. This seems
sufficiently unpleasant to justify back-patching this change.

Branch
------
REL9_0_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/b2f6754d2195dda8d7a1ec767ba3c2568458b524

Modified Files
--------------
contrib/uuid-ossp/uuid-ossp.c | 74 ++++++++++++++++++++++++-----------------
1 file changed, 44 insertions(+), 30 deletions(-)