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.
Merge pull request #64 from Zentyal/ejhernandez/store-change-number-subfolders-cache

Store changeNumber maps with modseq in cache for subfolders