Implement a model of resource ownership which allows apps to be used as context managers, triggering resource teardown on close

[sirosen]

Introduce the '_resource_owner' internal protocol

Several resources in the SDK now define a tracking protocol to allow
resources to participate in a cascading close() via a GlobusApp.

The design breaks down into two major sections.

Ownership Tracking

• _resource_owner: BaseClient | None is a new property of transports
• _resource_owner: GlobusApp | None is a new property of token storage
• _resource_owner: GlobusApp | None is a new property of clients
• _owned_clients: MutableSet[BaseClient] is a new property of apps

These are the storage mechanisms being used. The _owned_clients set is
defined as a weakref.WeakSet, meaning that membership in the set will
not prevent garbage collection of the members (which implicitly removes
them).

Each resource "may know its owner" by having the _resource_owner
assigned, and apps keep tabs on "owned clients" (which are otherwise
not associated).

The rules for when these are set are as follows:

• When an app is attached to a client (as through init) it is stored as
  _resource_owner
• When an app is attached to a client, the client is added to the app's
  _owned_clients
• When a client creates a transport, it sets itself as the transport's
  _resource_owner
• When an app creates a token storage, it sets itself as the token
  storage's _resource_owner

Close Semantics

• All token storages now define close(), but it is still only
  meaningful for some -- the base version is a no-op
• BaseClient._close() is a new (private) method which closes the
  underlying transport IFF the client is the owner
• GlobusApp.close() is a new (public) method which closes (_close())
  all apps owned by the client and any token storage owned by the client
• GlobusApp supports use as a context manager, with close() on exit

---

• Introduce the '_resource_owner' internal protocol
• Add unit tests for resource ownership + close
• Make User Guide use GlobusApp context managers
• Add explicit narrative doc for GlobusApp.close()

[sirosen]

This branch of work was continued, literally (commits preserved) in #1326