Converting isc_log to RWLOCK broke Windows

The !3229 (merged) broke Windows system tests - the tests are stuck indefinitely. See https://gitlab.isc.org/isc-projects/bind9/-/jobs/777703 (ondrej/msvc-stuck-v1 branch) as example vs https://gitlab.isc.org/isc-projects/bind9/-/jobs/777784 (ondrej/msvc-stuck-v2 branch).

There's nothing really obvious, but converting the rwlocks to native Windows SRWLock doesn't help, so it must be a way how we are using the locks on Windows.

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