FLUME-3133 Add client IP / hostname headers to Syslog sources.

[turcsanyip]

In the newer version of the Syslog message format (RFC-5424) the hostname
is not a mandatory header anymore so the Syslog client might not send it.
On the Flume side it would be a useful information that could be used
in interceptors or for event routing.
To keep this information, two new properties have been added to the Syslog
sources: clientIPHeader and clientHostnameHeader.
Flume users can define custom event header names through these parameters
for storing the IP address / hostname of the Syslog client in the Flume
event as headers.
The IP address / hostname are retrieved from the underlying network sockets,
not from the Syslog message.

This change is based on the patch submitted by Jinjiang Ling which has been
rebased onto the current trunk and the review comments have been implemented.

[szaboferee]

Thanks @turcsanyip for the contribution,

it looks good, however, the tests are failing:

2018-11-08 23:52:05,824 (main) [INFO - org.apache.flume.source.MultiportSyslogTCPSource.start(MultiportSyslogTCPSource.java:214)] Multiport Syslog TCP source null started.

java.net.ConnectException: Connection refused (Connection refused)

	at java.net.PlainSocketImpl.socketConnect(Native Method)
	at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
	at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
	at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
	at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
	at java.net.Socket.connect(Socket.java:589)
	at java.net.Socket.connect(Socket.java:538)
	at java.net.Socket.<init>(Socket.java:434)
	at java.net.Socket.<init>(Socket.java:244)
	at org.apache.flume.source.TestMultiportSyslogTCPSource.testClientHeaders(TestMultiportSyslogTCPSource.java:559)

I will try to get more details.

[szaboferee]

context.put("host", InetAddress.getLoopbackAddress().getHostAddress());
and

InetAddress localHost = InetAddress.getLocalHost();
Socket syslogSocket = new Socket(localHost, port);

are not the same. getLocalHost() is a different IP

[szaboferee]

Tests are ok.
+1

[majorendre]

LGTM, +1, ship it.