This change introduces system.clients table, which provides
information about CQL clients connected.
PK is the client's IP address, CK consists of outgoing port number
and client_type (which will be extended in future to thrift/alternator/redis).
Table supplies also shard_id and username. Other columns,
like connection_stage, driver_name, driver_version...,
are currently empty but exist for C* compatibility and future use.
This is an ordinary table (i.e. non-virtual) and it's updated upon
accepting connections. This is also why C*'s column request_count
was not introduced. In case of abrupt DB stop, the table should not persist,
so it's being truncated on startup.
Resolves#4820
36ec43a262