Replace the HELIX_ENABLED config with InstanceOperation while maintaining backwards compatibility with old APIs

Issues

[x] Deprecate the HELIX_ENABLED field in InstanceConfig and unify its functionality with InstanceOperation.

Description

In order to unify HELIX_ENABLED functionality with InstanceOperation, InstanceOperation will now have the following options:

ENABLE: Node can host online replicas and is assignable
DISABLE: Node cannot host online replicas but can be assigned replicas
EVACUATE: Node can host online replicas but cannot be assigned new replicas
SWAP_IN: Node is being swapped in and can host online replicas but is not assigned replicas by the rebalancer. The mirroring is handled outside the rebalancer. SWAP_IN nodes are not populated in the RoutingTableProvider.
UNKNOWN: Node cannot host online replicas, is not assignable, and is not populated in the RoutingTableProvider.

SWAP_IN will not be able to be set to DISABLE, since the SWAP_IN instance is not in the routing table it should not require a graceful way to move replicas to OFFLINE state.

Tests

[x] Adjusted tests to use new APIs and kept tests using old APIs to ensure backwards compatibility

Changes that Break Backward Compatibility (Optional)

Will maintain backwards compatibility.

To replace the HELIX_ENABLED config with InstanceOperation while maintaining backwards compatibility with old APIs we keep the behavior where helix will treat the instance as disabled if HELIX_ENABLED is set to false with the exception of SWAP_IN. If it is set to true, the controller will treat the instance based on the InstanceOperation that is set. All users on an old version of helix will be able to continue using HelixAdmin().enableInstance with the same behavior. Users on the latest version will be able to leverage the new APIs to do new operations available.

Commits

My commits all reference appropriate Apache Helix GitHub issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
1. Subject is separated from body by a blank line
2. Subject is limited to 50 characters (not including Jira issue reference)
3. Subject does not end with a period
4. Subject uses the imperative mood ("add", not "adding")
5. Body wraps at 72 characters
6. Body explains "what" and "why", not "how"

Code Quality

My diff has been formatted using helix-style.xml (helix-style-intellij.xml if IntelliJ IDE is used)

apache / helix