fix a TSAN bug in "rndc fetchlimit"
fctx counters could be accessed without locking when "rndc fetchlimit" is called; while this is probably harmless in production, it triggered TSAN reports in system tests.
fctx counters could be accessed without locking when "rndc fetchlimit" is called; while this is probably harmless in production, it triggered TSAN reports in system tests.