Shm UR ref count segment: fallback to old behaviour when segment size is set to 0

[rbx]

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Mention any issue this PR resolves or is related to.

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Checklist:

• Rebased against dev branch
• My name is in the resp. CONTRIBUTORS/AUTHORS file
• Followed the seven rules of great commit messages

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Walkthrough

Walkthrough

The changes across various files indicate a significant update to the shared memory management, particularly in handling reference counting for unmanaged regions. A new configuration parameter rcSegmentSize has been introduced, which is set to 0 to denote unmanaged regions. The logic for reference counting in messages has been updated to accommodate this new parameter, affecting how memory is allocated, copied, and released. The RegionInfo structure and related constructors have been simplified by removing default segment size constants and parameters.

Changes

File Path	Change Summary
examples/.../CMakeLists.txt	Modified add_example function to include 'region-advanced-external' argument.
examples/.../fairmq-start-ex-region-advanced.sh.in	Added --rc-segment-size 0 flag to SAMPLER configuration.
examples/.../keep-alive.cxx	Altered ShmManager logic to set cfg.rcSegmentSize to 0 and pass cfg to UnmanagedRegion constructor.
fairmq/shmem/Common.h	Removed DefaultRcSegmentSize constant and related constructors from RegionInfo.
fairmq/shmem/Message.h	Updated GetRefCount(), Copy(), and Release() methods to handle new rcSegmentSize logic.
fairmq/shmem/UnmanagedRegion.h	Added fRcSegmentSize field and adjusted initialization logic for reference counting.
test/message/_message.cxx	Updated ZeroCopyFromUnmanaged() to handle new rcSegmentSize parameter and added related test cases.

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