CQL Management API

We want to run management commands via CQL.
The goals are:

• To provide a way to run predefined management commands via CQL;
• To provide a mechanism for retrieving command definitions and metadata via CQL;
• To provide information on all available management commands via virtual tables;
• To provide a registry that stores all C* commands and their metadata accordingly;
• To internal instrumentation and a reasonable plan for migrating cluster management from JMX to CQL, taking into account backward compatibility and adopted deprecation policies;

The discussion on the ML:
https://lists.apache.org/thread/pow83q92m666nqtwyw4m3b18nnkgj2y8

The design document:
https://cwiki.apache.org/confluence/display/CASSANDRA/CEP-38%3A+CQL+Management+API