Implement merging and syncing items in PodClient

• for get, update, search, bulk: All items are now stored in the local_db, with the following logic:

if item.id not in local_db:
    local_db.add(item)
else:
    local_item = local_db[item.id]
    merge(local_item, item, merge_priority)

• add requires_client_ref to ItemBase to keep compatible with Dataset
• ItemBase has an ensure_id method, that creates a new ID if the item doesnt have one.
• ItemBase now has an _in_pod property, which tracks if the item already exists in pod.
• ItemBase now tracks which properties are updated by keeping a record of original_properties and date_local_modified. Each changed property gets recorded in both, which enables local, remote and newest merge strategies
• ItemBase now has an on_sync() method, which resets these tracking variables to the appropriate values after syncing.