fix: dev: Fix the memory ordering in the adaptive read-write lock

On hardware with a weak memory model, the internal read-write lock could
briefly admit a reader and a writer at the same time, risking sporadic
crashes or incorrect data. The reader/writer handshake now uses
sequentially consistent ordering so the two can no longer overlap.

Closes #6060

Merge branch '6060-rwlock-seq-cst-handshake' into 'main'

See merge request isc-projects/bind9!12162