generic_server: add on_connection_ready handler

This patch cleans the code a bit so that ready state is set in a single place.
And adds handler which will allow adding logic when connection is made
ready, this will be added in the following commits.

generic_server.cc

@@ -129,6 +129,10 @@ future<> connection::process()
     });
 }
 
+void connection::on_connection_ready()
+{
+}
+
 void connection::on_connection_close()
 {
 }

generic_server.hh

@@ -62,6 +62,8 @@ public:
 
     virtual future<> process_request() = 0;
 
+    virtual void on_connection_ready();
+
     virtual void on_connection_close();
 
     virtual future<> shutdown();

redis/server.cc

@@ -108,6 +108,7 @@ future<> redis_server::connection::process_request() {
         _pending_requests_gate.enter();
         utils::latency_counter lc;
         lc.start();
+        on_connection_ready();
         auto leave = defer([this] () noexcept { _pending_requests_gate.leave(); });
         return process_request_internal().then([this, leave = std::move(leave), lc = std::move(lc)] (auto&& result) mutable {
             --_server._stats._requests_serving;

transport/server.cc

@@ -934,6 +934,7 @@ future<std::unique_ptr<cql_server::response>> cql_server::connection::process_st
         }
     } else {
         _ready = true;
+        on_connection_ready();
         res = make_ready(stream, trace_state);
     }
 
@@ -967,6 +968,7 @@ future<std::unique_ptr<cql_server::response>> cql_server::connection::process_au
                 return f.then([this, stream, challenge = std::move(challenge), trace_state]() mutable {
                     _authenticating = false;
                     _ready = true;
+                    on_connection_ready();
                     return make_ready_future<std::unique_ptr<cql_server::response>>(make_auth_success(stream, std::move(challenge), trace_state));
                 });
             });
@@ -1324,6 +1326,7 @@ cql_server::connection::process_register(uint16_t stream, request_reader in, ser
         _server._notifier->register_event(et, this);
     }
     _ready = true;
+    on_connection_ready();
     return make_ready_future<std::unique_ptr<cql_server::response>>(make_ready(stream, std::move(trace_state)));
 }