Improve throughput performance #2 (refcount focus)

[jean-roland]

This PR continues on the work of #730. Profiling helped identify multiple causes of performance issue, including our refcount implementation with weak pointers.

• More static inline rework: Similar work to what was done previously.

• Lazier & faster clear / initialization: Some types were taking time initializing or clearing things that weren't necessary. They got simplified. Chief of this was timestamps. A validity boolean flag was added to the struct to simplify checks & clears of the type.

• "Simple rc" - name subject to change: Added back a rc implementation without weak, used only for arc_slices. Closes #520

• Add Z_FEATURE_PUBLISHER_SESSION_CHECK config token On by default, this can be turned off to avoid upgrading the publisher's session weak in apps where session is not dropped before it stops publishing.

• Reworked logs from the transport/codec layer.

• Multicast read task more tolerant: As pico multicast is on a best-effort transport only, a missed fragment shouldn't close the connection (it was causing tests to halt).

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