Thread API cleanup

This patch cleans up the thread API and comes with the following
noteworthy changes:

- Introduced Cpu_session::Weight type that replaces a formerly used
  plain integer value to prevent the accidental mix-up of
  arguments.
- The enum definition of Cpu_session::DEFAULT_WEIGHT moved to
  Cpu_session::Weight::DEFAULT_WEIGHT
- New Thread constructor that takes a 'Env &' as first argument.
  The original constructors are now marked as deprecated. For the
  common use case where the default 'Weight' and 'Affinity' are
  used, a shortcut is provided. In the long term, those two
  constructors should be the only ones to remain.
- The former 'Thread<>' class template has been renamed to
  'Thread_deprecated'.
- The former 'Thread_base' class is now called 'Thread'.
- The new 'name()' accessor returns the thread's name as 'Name'
  object as centrally defined via 'Cpu_session::Name'. It is meant to
  replace the old-fashioned 'name' method that takes a buffer and size
  as arguments.
- Adaptation of the thread test to the new API

Issue #1954

repos/dde_rump/include/util/hard_context.h

@@ -54,7 +54,7 @@ class Hard_context
 
 
 class Hard_context_thread : public Hard_context,
-                            public Genode::Thread<sizeof(Genode::addr_t) * 2048>
+                            public Genode::Thread_deprecated<sizeof(Genode::addr_t) * 2048>
 {
 	private:
 
@@ -72,7 +72,7 @@ class Hard_context_thread : public Hard_context,
 	public:
 
 		Hard_context_thread(char const *name, func f, void *arg, int cookie, bool run = true)
-		: Hard_context(cookie), Thread(name),
+		: Hard_context(cookie), Thread_deprecated(name),
 			_func(f), _arg(arg) { if (run) start(); }
 };