The completed and received counters must work in tandem and the code
should better reflect that. Previously, the act of updating them was 2-3
lines of code duplicated in multiple places. This consolidates them into
a single update closure for easy reading and maintenance.
This also simplifies error handling in places where we can use a return
parameter and defer to handle the error case for updates.
Also, remove the old Layer field from the trackingReader struct.
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