Issue #84: Fix NPE while enable broker publish throttling

[jiazhai]

Fix #84

while set

brokerPublisherThrottlingMaxByteRate

it will meet error:

08:38:34.452 [pulsar-broker-publish-rate-limiter-monitor-30-1] ERROR org.apache.bookkeeper.common.util.SafeRunnable - Unexpected throwable caught
java.lang.NullPointerException: null
        at org.apache.pulsar.broker.service.ServerCnx.enableCnxAutoRead(ServerCnx.java:1605) ~[org.apache.pulsar-pulsar-broker-2.5.0.jar:2.5.0]
        at org.apache.pulsar.broker.service.AbstractTopic.lambda$enableProducerRead$5(AbstractTopic.java:295) ~[org.apache.pulsar-pulsar-broker-2.5.0.jar:2.5.0]
        at java.util.concurrent.ConcurrentHashMap$ValuesView.forEach(ConcurrentHashMap.java:4707) ~[?:1.8.0_242]
        at org.apache.pulsar.broker.service.AbstractTopic.enableProducerRead(AbstractTopic.java:295) ~[org.apache.pulsar-pulsar-broker-2.5.0.jar:2.5.0]
        at org.apache.pulsar.broker.service.AbstractTopic.resetBrokerPublishCountAndEnableReadIfRequired(AbstractTopic.java:286) ~[org.apache.pulsar-pulsar-broker-2.5.0.jar:2.5.0]
        at org.apache.pulsar.broker.service.BrokerService.lambda$refreshBrokerPublishRate$39(BrokerService.java:1138) ~[org.apache.pulsar-pulsar-broker-2.5.0.jar:2.5.0]
        at org.apache.pulsar.broker.service.BrokerService.lambda$forEachTopic$40(BrokerService.java:1148) ~[org.apache.pulsar-pulsar-broker-2.5.0.jar:2.5.0]
        at org.apache.pulsar.common.util.collections.ConcurrentOpenHashMap$Section.forEach(ConcurrentOpenHashMap.java:385) ~[org.apache.pulsar-pulsar-common-2.5.0.jar:2.5.0]
        at org.apache.pulsar.common.util.collections.ConcurrentOpenHashMap.forEach(ConcurrentOpenHashMap.java:159) ~[org.apache.pulsar-pulsar-common-2.5.0.jar:2.5.0]
        at org.apache.pulsar.broker.service.BrokerService.forEachTopic(BrokerService.java:1145) ~[org.apache.pulsar-pulsar-broker-2.5.0.jar:2.5.0]
        at org.apache.pulsar.broker.service.BrokerService.refreshBrokerPublishRate(BrokerService.java:1138) ~[org.apache.pulsar-pulsar-broker-2.5.0.jar:2.5.0]
        at org.apache.pulsar.broker.service.BrokerService.lambda$setupBrokerPublishRateLimiterMonitor$9(BrokerService.java:509) ~[org.apache.pu

This is caused by the mock Producer that we registered for bundle load/unload handling.
change: provide a mock method in InternalServerCnx.