Problem: duplicate but equivalent state enums in curve_server_t and plain_server_t

Solution: pull state enum up to zap_client_t and unify names of enum values

src/plain_server.hpp

@@ -57,17 +57,6 @@ namespace zmq
 
     private:
 
-        enum state_t {
-            waiting_for_hello,
-            sending_welcome,
-            waiting_for_initiate,
-            sending_ready,
-            waiting_for_zap_reply,
-            sending_error,
-            error_command_sent,
-            ready
-        };
-
         state_t state;
 
         int produce_welcome (msg_t *msg_) const;