Add several fixes, enhancements and features in Mria:
  - protect `mria:join/1,2` with a global lock to prevent conflicts between
    two nodes trying to join each other simultaneously
    [Mria PR](https://github.com/emqx/mria/pull/137)
  - implement new function `mria:sync_transaction/4,3,2`, which blocks the caller until
    a transaction is imported to the local node (if the local node is a replicant, otherwise,
    it behaves exactly the same as `mria:transaction/3,2`)
    [Mria PR](https://github.com/emqx/mria/pull/136)
  - optimize `mria:running_nodes/0`
    [Mria PR](https://github.com/emqx/mria/pull/135)
  - optimize `mria:ro_transaction/2` when called on a replicant node
    [Mria PR](https://github.com/emqx/mria/pull/134).