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/base-nova/src/lib/base/stack.cc

@@ -109,11 +109,11 @@ void prepare_reinit_main_thread()
 }
 
 
-/*****************
- ** Thread_base **
- *****************/
+/************
+ ** Thread **
+ ************/
 
-Native_utcb *Thread_base::utcb()
+Native_utcb *Thread::utcb()
 {
 	/*
 	 * If 'utcb' is called on the object returned by 'myself',