callagent: Add setter for the MGCPEndpoint

Verify the state where the SDP file and the CallId can be
set. Unset those when the endpoint is put back into the unused
state.

callagent/MGCPEndpoint.st

@@ -92,6 +92,8 @@ Object subclass: MGCPEndpoint [
         <category: 'allocation'>
         self requireState: self class stateUsed.
         state := self class stateUnused.
+        sdp := nil.
+        callid := nil.
     ]
 
     tryBlock [
@@ -117,6 +119,17 @@ Object subclass: MGCPEndpoint [
 
     sdp: aSdp [
         <category: 'sdp'>
+        self requireState: self class stateUsed.
         sdp := aSdp.
     ]
+
+    callId [
+        <category: 'callid'>
+        ^ callid
+    ]
+
+    callId: aCallId [
+        self requireState: self class stateReserved.
+        callid := aCallId.
+    ]
 ]