Store changeNumber maps with modseq in cache for subfolders

By getting the root folder/container whose properties
are stored in OpenChange DB.

This makes the synchronisation of sub-folders faster as
when we evaluate restrictions for this folder, we are able
to get the modseq from where to get the latest messages
unseen by the client.

OpenChange/MAPIStoreFolder.h

@@ -150,6 +150,7 @@
 
 /* helpers */
 - (uint64_t) idForObjectWithKey: (NSString *) childKey;
+- (MAPIStoreFolder *) rootContainer;
 
 /* subclasses */
 - (MAPIStoreMessage *) createMessage;

OpenChange/MAPIStoreFolder.m

@@ -1885,6 +1885,17 @@ Class NSExceptionK, MAPIStoreFAIMessageK, MAPIStoreMessageTableK, MAPIStoreFAIMe
                                 inFolderURL: [self url]];
 }
 
+- (MAPIStoreFolder *) rootContainer
+{
+  /* Return the oldest ancestor, which does not have
+     container. If there is not container, it returns itself.
+  */
+  if (container)
+    return [container rootContainer];
+  else
+    return self;
+}
+
 - (NSDate *) creationTime
 {
   return [dbFolder creationDate];

OpenChange/MAPIStoreMailFolder.m

@@ -682,9 +682,9 @@ _compareFetchResultsByMODSEQ (id entry1, id entry2, void *data)
 {
   /* This method should be called whenever something has changed on the folder.
      Then we will perform two actions:
-         1 - Update its PidTagChangeNumber property.
+         1 - Update the PidTagChangeNumber property of the root container.
          2 - Store relationship PidTagChangenumber with lastModseq value on the
-             mapping given as parameter */
+             mapping given as parameter for this folder */
   uint64_t *current_cn;
   struct SRow row;
   struct SPropValue prop;
@@ -699,10 +699,11 @@ _compareFetchResultsByMODSEQ (id entry1, id entry2, void *data)
   prop.value.d = 0; // It doesn't matter, it will be autogenerated
   row.lpProps = ∝
 
-  /* We are doing a "touch" operation to update change number of this folder */
+  /* We are doing a "touch" operation to update change number of the root container.
+     We get the root container as it has the properties in the OpenChange DB */
   username = [[self context] connectionInfo]->username;
   oc_ctx = [[self context] connectionInfo]->oc_ctx;
-  fid = [self objectId];
+  fid = [[self rootContainer] objectId];
   retval = openchangedb_set_folder_properties(oc_ctx, username, fid, &row);
   if (retval != MAPI_E_SUCCESS)
     {