journal: normalize priority of logging sources

The stream event source has a priority of SD_EVENT_PRIORITY_NORMAL+5,
and stdout source +10, but the native and syslog event sources are left
at the default of 0.

As a result, any heavy native or syslog logger can cause starvation of
the other loggers.  This is trivially demonstrated by running:

 dd if=/dev/urandom bs=8k | od | systemd-cat & # native spammer
 systemd-run echo hello & # stream logger
 journalctl --follow --output=verbose --no-pager --identifier=echo &

... and wait, and wait, the "hello" never comes.

Now kill %1, "hello" arrives finally.

src/journal/journald-native.c

@@ -495,5 +495,9 @@ int server_open_native_socket(Server*s) {
         if (r < 0)
                 return log_error_errno(r, "Failed to add native server fd to event loop: %m");
 
+        r = sd_event_source_set_priority(s->native_event_source, SD_EVENT_PRIORITY_NORMAL+5);
+        if (r < 0)
+                return log_error_errno(r, "Failed to adjust native event source priority: %m");
+
         return 0;
 }

src/journal/journald-stream.c

@@ -733,7 +733,7 @@ int server_open_stdout_socket(Server *s) {
         if (r < 0)
                 return log_error_errno(r, "Failed to add stdout server fd to event source: %m");
 
-        r = sd_event_source_set_priority(s->stdout_event_source, SD_EVENT_PRIORITY_NORMAL+10);
+        r = sd_event_source_set_priority(s->stdout_event_source, SD_EVENT_PRIORITY_NORMAL+5);
         if (r < 0)
                 return log_error_errno(r, "Failed to adjust priority of stdout server event source: %m");
 

src/journal/journald-syslog.c

@@ -430,6 +430,10 @@ int server_open_syslog_socket(Server *s) {
         if (r < 0)
                 return log_error_errno(r, "Failed to add syslog server fd to event loop: %m");
 
+        r = sd_event_source_set_priority(s->syslog_event_source, SD_EVENT_PRIORITY_NORMAL+5);
+        if (r < 0)
+                return log_error_errno(r, "Failed to adjust syslog event source priority: %m");
+
         return 0;
 }