Add the following fixes and features in Mria:
 - call `mria_rlog:role/1` safely in mria_membership to ensure that mria_membership
   gen_server won't crash if RPC to another node fails
   [Mria PR](https://github.com/emqx/mria/pull/139)
 - Add extra field to ?rlog_sync table to facilitate extending this functionality in future
   [Mria PR](https://github.com/emqx/mria/pull/138).