Migrate cluster table to new watch-based APIs

[kralicky]

New APIs implemented in #643
POST /management/clusters/watch -> will stream create/update/delete events as clusters are modified in real time, starting with create events for all clusters that currently exist
POST /management/clusters/{id}/health/watch -> will stream live health/status updates for the cluster with the given ID, starting with the cluster's current health/status.

[kralicky]

Moved to #1679