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/include/signal_source/client.h

@@ -52,7 +52,7 @@ namespace Genode {
 			: Rpc_client<Nova_signal_source>(static_cap_cast<Nova_signal_source>(cap))
 			{
 				/* request mapping of semaphore capability selector */
-				Thread_base * myself = Thread_base::myself();
+				Thread * myself = Thread::myself();
 				request_signal_sm_cap(Native_capability(myself->native_thread().ec_sel + 1),
 				                      myself->native_thread().exc_pt_sel + Nova::PT_SEL_STARTUP);
 				_sem = Native_capability(myself->native_thread().exc_pt_sel + Nova::PT_SEL_STARTUP);