oc: remove and disable userContext after each call

This adds [MAPIStoreUserContext activate] method to use
it instead of activateWithUser.

A cleanup operation is executed after each public function
so there won't be any conflicts with future calls.

In practice, this will deactivate the current user context set on
MAPIApp, this means two things: (1) set nil as current user context
on MAPIApp and (2) remove woContext from current thread dictionary

OpenChange/MAPIApplication.m

@@ -76,6 +76,11 @@ MAPIApplication *MAPIApp = nil;
   return NO;
 }
 
+- (MAPIStoreUserContext *) userContext
+{
+    return userContext;
+}
+
 - (void) setUserContext: (MAPIStoreUserContext *) newContext
 {
   /* user contexts must not be retained here ad their holder (mapistore)
@@ -88,4 +93,9 @@ MAPIApplication *MAPIApp = nil;
   return [userContext authenticator];
 }
 
+- (void) cleanup
+{
+  [userContext deactivate];
+}
+
 @end