### Motivation

For a multi-topics consumer, when it acknowledges a single message, it
will first find the owner topic from its message ID. If the owner topic
is not subscribed by the consumer, `NotConnectedException` will be
thrown.

However, when it acknowledges multiple messages, if any of them is the
message whose owner topic is not subscribed by the consumer, NPE will
happen instead.

### Modifications

When acknowledging multiple messages, ignore the message IDs whose owner
topic is not subscribed. `testAckMessageInAnotherTopic` is added to
cover this case.

### TODO

There are many other places that do not check if `consumers.get` returns
`null`, like `doReconsumeLater`, `negativeAcknowledge`, etc. This patch
does not cover them.