GSUP: implement TCAP-like session management

Unlike TCAP/MAP, GSUP is just a transport layer without the
dialogue/context. This prevents us from having session based
communication, required e.g. for USSD. But we can emulate
TCAP dialogue by adding additional IEs, which would allow
to relate each message to a particular session.

This change introduces the following IEs:

  - OSMO_GSUP_SESSION_ID_IE,
  - OSMO_GSUP_SESSION_STATE_IE,

which optionally can be used to indicate that the message is
related to a session with given ID, and to manage session
state, i.e. initiate, continue, and finish.

Change-Id: I1cee271fed0284a134ffed103c0d4bebbcfde2a8
Related: OS#1597
diff --git a/TODO-RELEASE b/TODO-RELEASE
index 8ccfa49..d984813 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -7,3 +7,5 @@
 # If any interfaces have been added since the last public release: c:r:a + 1.
 # If any interfaces have been removed or changed since the last public release: c:r:0.
 #library	what			description / commit summary line
+gsup		gsup.h				the 'osmo_gsup_message' struct extended with
+						session information => ABI changed