Fix the data race when shutting down dns_adb

When dns_adb is shutting down, first the adb->shutting_down flag is set
and then task is created that runs shutdown_stage2() that sets the
shutdown flag on names and entries.  However, when dns_adb_createfind()
is called, only the individual shutdown flags are being checked, and the
global adb->shutting_down flag was not checked.  Because of that it was
possible for a different thread to slip in and create new find between
the dns_adb_shutdown() and dns_adb_detach(), but before the
shutdown_stage2() task is complete.  This was detected by
ThreadSanitizer as data race because the zonetable might have been
already detached by dns_view shutdown process and simultaneously
accessed by dns_adb_createfind().

This commit converts the adb->shutting_down to atomic_bool to prevent
the global adb lock when creating the find.
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