LU-10499 pcc: introducing OBD_CONNECT2_PCCRO flag

LU-10499 pcc: introducing OBD_CONNECT2_PCCRO flag

Add a new connection flag OBD_CONNECT2_PCCRO to solve the access
consistency from the old client without PCC-RO support.

By necessity, also include definitions for OBD_CONNECT2_MODE_CONVERT
and OBD_CONNECT2_BATCH_RPC so obd_connect_names[] works.

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I19716e94a86e53353c1628d414c92e61e084dfc9
Reviewed-on: https://review.whamcloud.com/40791
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>