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/session/session.h

@@ -21,13 +21,13 @@
  */
 #include <base/rpc.h>
 
-namespace Genode { class Session; }
+namespace Genode { struct Session; }
 
 
 /**
  * Base class of session interfaces
  */
-class Genode::Session
+struct Genode::Session
 {
 	/*
 	 * Each session interface must implement the class function 'service_name'
@@ -35,6 +35,8 @@ class Genode::Session
 	 * This function returns the name of the service provided via the session
 	 * interface.
 	 */
+
+	virtual ~Session() { }
 };
 
 #endif /* _INCLUDE__SESSION__SESSION_H_ */