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ARROW-2558: [Plasma] avoid walk through all the objects when a client disconnects

Currently plasma stores list-of-clients in ObjectTableEntry, which is used to track which clients are using a given object, this serves for two purposes:
- If an object is in use.
- If the client trying to abort an object is the one who created it.

A problem with list-of-clients approach is that when a client disconnects, we need to walk through all the objects and remove the client pointer from the list for each object.

Instead, we could add a reference count in ObjectTableEntry, and store list-of-object-ids in client structure. This could both goals that the original approach is targeting, while when a client disconnects, it just walk through its object-ids and dereference each ObjectTableEntry, there's no need to walk through all objects.

Author: Zhijun Fu <zhijun.fu@outlook.com>

Closes #2015 from zhijunfu/client_object_ids and squashes the following commits:

d8db8f75 <Zhijun Fu> Address comments from pcmoritz
8a439e88 <Zhijun Fu> Trigger
a0475725 <Zhijun Fu>  use list-of-object-ids in client instead of list-of-clients in object