TS-2259: Introduce failover hosts for logging system

This patch introduce failover hosts for collation host by using '|' delimiter
to extend <CollationHosts> tags in logs_xml.config.

Let me give an example:
<CollationHosts = "host1:5000|host2:5000|host3:6000, 209.131.52.129:6000"/>

In the example above, host2/host3 are failover hosts for host1. When host1
disconnected, log entries will be sent to host2, and then if host2 failed again,
log entries will be sent to host3 until host1 or host2 comes back.

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>
TS-2260: Avoid flooding log when log host is down

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>
Add CHANGES for TS-2259/TS-2260

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>