core: Introduce 'Address_space' interface

The new core-internal 'Address_space' interface enables cores RM service
to flush mappings of a PD in which a given 'Rm_client' thread resides.
Prior this patch, each platform invented their own way to flush mappings
in the respective 'rm_session_support.cc' implementation. However, those
implementations used to deal poorly with some corner cases. In
particular, if a PD session was destroyed prior a RM session, the RM
session would try to use no longer existing PD session. The new
'Address_space' uses the just added weak-pointer mechanism to deal with
this issue.

Furthermore, the generic 'Rm_session_component::detach' function has
been improved to avoid duplicated unmap operations for platforms that
implement the 'Address_space' interface. Therefore, it is related to
issue #595. Right now, this is OKL4 only, but other platforms will follow.

base/include/rm_session/rm_session.h

@@ -100,6 +100,7 @@ namespace Genode {
 		class Out_of_metadata   : public Attach_failed { };
 
 		class Invalid_thread    : public Exception { };
+		class Unbound_thread    : public Exception { };
 
 		/**
 		 * Destructor
@@ -156,6 +157,7 @@ namespace Genode {
 		 * \param thread  thread that will be paged
 		 * \throw         Invalid_thread
 		 * \throw         Out_of_metadata
+		 * \throw         Unbound_thread
 		 * \return        capability to be used for handling page faults
 		 *
 		 * This method must be called at least once to establish a valid