Cache platform_and_version and hostname

These values don't chnage at runtime and
take a surprising amount of resources when
opening a lot of connections. This speeds
up opening many connections/channels by 10%
in my tests.

Author: mkuratczyk

deps/rabbit_common/src/rabbit_misc.erl

@@ -1057,7 +1057,14 @@ otp_release() ->
     end.
 
 platform_and_version() ->
-    string:join(["Erlang/OTP", otp_release()], " ").
+    case persistent_term:get(platform_and_version, undefined) of
+        undefined ->
+            PV = string:join(["Erlang/OTP", otp_release()], " "),
+            persistent_term:put(platform_and_version, PV),
+            PV;
+        PV ->
+            PV
+    end.
 
 otp_system_version() ->
     string:strip(erlang:system_info(system_version), both, $\n).

deps/rabbit_common/src/rabbit_net.erl

@@ -237,12 +237,19 @@ tcp_host(IPAddress) ->
     end.
 
 hostname() ->
-    {ok, Hostname} = inet:gethostname(),
-    case inet:gethostbyname(Hostname) of
-        {ok,    #hostent{h_name = Name}} -> Name;
-        {error, _Reason}                 -> Hostname
+    case persistent_term:get(platform_and_version, undefined) of
+        undefined ->
+            {ok, Hostname} = inet:gethostname(),
+            H = case inet:gethostbyname(Hostname) of
+                    {ok,    #hostent{h_name = Name}} -> Name;
+                    {error, _Reason}                 -> Hostname
+                end,
+                persistent_term:put(platform_and_version, H);
+        Hostname ->
+            Hostname
     end.
 
+
 format_nic_attribute({Key, undefined}) ->
     {Key, undefined};
 format_nic_attribute({Key = flags, List}) when is_list(List) ->