Clear unused stuff from PgDtc_isolate() function. Also add CC_initialize_pg_version() call after CC_copy_conninfo() so as to avoid the trouble caused by early PG_VERSION_xx() calls.
Improve pgxalib.dll(MSDTC support).
1. Change xa_open() so that it tries to connect to the database immediately.
   This would cause an error at enlistment in dtc when the authentication
   method depends not only on the db user but also on the OS user and it
   means the automatic transaction recovery is unavailable.
2. Log the error message in case of SQLDriverConnect() error.
3. Improve the logging.