Address the delta review comments from kafka-1227.
20. Selector and Sender: need to close socket channel on IOException in addition to UnresolvedHostnameException.
21.1 To be consistent, we probably should change the return value of the following api from Long to long.
public Long getLong(String key)
21.1 Could we also add getDouble(String key)?
22. Metadata.fetch(): The wait time is incorrect when there is an InterruptedException.
23. Add comments on BufferPool.allocate() and Struct.instanceOf() for clarification.
24. Various other minor fixes for mis-spelling.