properly sequenced object shutdown for modules using Python client

[d-w-moore]

(PRC = Python iRODS Client)
Some modules (Example: iRODFS) may leverage PRC-level objects, but possibly sit at a different (likely higher) abstraction level , and so it would be common for these modules' authors to prefer that their objects finalization should occur as prelude or perhaps even postlude to PRC objects' shutdown.

in iRODSFS's case for example, their handles are higher-level and should be closed first, before PRC internal shutdown functions execute.
(PRC internally shuts down network connections in certain situations, e.g. to guarantee saving of data object content written but not yet flushed, and to return data objects to an unlocked status.)
We should provide an interface to guarantee all of this can happen in the proper sequence.

Example
from some_module_using_prc:

• irods.register_function_for_client_shutdown ( my_close_function , RUN_BEFORE_PYTHON_IRODSCLIENT_SHUTDOWN)