Logging now doesn't lock with each message

The original logging routines were locking a common mutex. This led to
massive underperformance and unwanted serialization when heavily logging
due to lock contention. Now the logging is lockless, though still
serializing on write() syscalls to the same filedescriptor.

This change also brings in a persistent logging channel structures and
thus avoids writing into active configuration data structures during
regular run.

lib/locking.h

@@ -21,6 +21,7 @@ struct lock_order {
   struct domain_generic *service;
   struct domain_generic *rtable;
   struct domain_generic *attrs;
+  struct domain_generic *logging;
   struct domain_generic *resource;
 };