- Upgrade ekka to 0.15.13, which includes the following changes:
  - upgrade Mria to 0.6.2:
    - make bootstrap batch size configurable, [Mria PR](https://github.com/emqx/mria/pull/159)
    - improve robustness of mria_membership processes, [Mria PR](https://github.com/emqx/mria/pull/156)
  - fix log message formatting error
- Add `node.default_bootstrap_batch_size` option to EMQX configuration.
Increasing the value of this  option can greatly reduce a replicant node startup time, especially when EMQX cluster interconnect network latency is high and EMQX built-in database holds large amount of data, e.g. when the number of subscriptions is high.