core: Make GUID encoders singletons

The GUID encoders used in LDAP operations are stateless and can be
reused, so we don't need to create a new instance each time we need
them. This patch makes them singletons.

Change-Id: If8683e90ac3a2a6c0ed129686c9e06ccb47ad82d
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>