Pull up the cql_server process() to base class and convert redis_server
to use it.
Please note that this fixes EPIPE and connection reset issue in the
Redis server, which was fixed in the CQL server in commit 1a8630e6a
("transport: silence "broken pipe" and "connection reset by peer"
errors").
The cql_server and redis_server classes have a maybe_idle() method,
which sets the _all_connections_stopped promise if server wants to stop
and can be stopped. Pull up the duplicated code to
generic_server::server class.
This patch moves the duplicated connection::shutdown() method to to a
new generic_server::connection base class that is now inherited by
cql_server and redis_server.
