ra_serf: clean up GSSAPI objects after use.
ra_serf: clean up GSSAPI objects after use.

* subversion/libsvn_ra_serf/auth_kerb.c
  (gss_api_get_credentials): release the two objects returned by
   gssapi calls.
  (cleanup_gss_ctx): New function.
  (svn_ra_serf__init_kerb_connection): Register cleanup_gss_ctx 
   to cleanup the gssapi context when the serf session is cleared
   or destroyed.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion