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/demo/src/lib/launchpad/launchpad.cc

@@ -295,7 +295,7 @@ Launchpad_child *Launchpad::start_child(const char *filename,
  * this case using a watchdog mechanism, unblock the 'close' call, and
  * proceed with the closing the other remaining sessions.
  */
-class Child_destructor_thread : Thread<2*4096>
+class Child_destructor_thread : Thread_deprecated<2*4096>
 {
 	private:
 
@@ -343,7 +343,7 @@ class Child_destructor_thread : Thread<2*4096>
 		 * Constructor
 		 */
 		Child_destructor_thread() :
-			Thread("child_destructor"),
+			Thread_deprecated("child_destructor"),
 			_curr_child(0), _curr_alloc(0),
 			_activate_lock(Lock::LOCKED),
 			_ready(true)