New Features and Improvements:

 - ClusterService: Add ClusterEvent#attach(...) method to allow execution of custom asynchronous tasks that get attached to the cluster event lifecycle.  - Core: Further improvements of internal state management of Hekate node

Bug fixes:

 - Cluster: Fix cluster event type filtering not being applied.

Dependency updates:

 - Spring Boot 2.1.15.RELEASE

Dev:

 - Migrate to Github Actions.  - Fix javadoc-related errors when building with JDK 11  - Remove redundant assertions from the source code