Remove blocking calls from root and parent RPCs

This is a redesign of the root and parent interfaces to eliminate
blocking RPC calls.

- New session representation at the parent (base/session_state.h)
- base-internal root proxy mechanism as migration path
- Redesign of base/service.h
- Removes ancient 'Connection::KEEP_OPEN' feature
- Interface change of 'Child', 'Child_policy', 'Slave', 'Slave_policy'
- New 'Slave::Connection'
- Changed child-construction procedure to be compatible with the
  non-blocking parent interface and to be easier to use
- The child's initial LOG session, its binary ROM session, and the
  linker ROM session have become part of the child's envirenment.
- Session upgrading must now be performed via 'env.upgrade' instead
  of performing a sole RPC call the parent. To make RAM upgrades
  easier, the 'Connection' provides a new 'upgrade_ram' method.

Issue #2120

repos/base/include/cpu_session/cpu_session.h

@@ -23,11 +23,20 @@
 #include <dataspace/capability.h>
 #include <pd_session/pd_session.h>
 
-namespace Genode { struct Cpu_session; }
+namespace Genode {
+
+	struct Cpu_session;
+	struct Cpu_session_client;
+}
 
 
 struct Genode::Cpu_session : Session
 {
+	static const char *service_name() { return "CPU"; }
+
+	typedef Cpu_session_client Client;
+
+
 	/*********************
 	 ** Exception types **
 	 *********************/
@@ -36,7 +45,6 @@ struct Genode::Cpu_session : Session
 	class Quota_exceeded         : public Thread_creation_failed { };
 	class Out_of_metadata        : public Exception { };
 
-	static const char *service_name() { return "CPU"; }
 
 	enum { THREAD_NAME_LEN = 32 };
 	enum { PRIORITY_LIMIT = 1 << 16 };