add standby namenode support

if the 'standby_namenode' directive has been set,
the plugin will automatically switch to use standby namenode under any
of the following scenes:
- Try to write the data to HDFS but got an 'org.apache.hadoop.ipc.StandbyException' exception
- HDFS-write failed >= 8 times
- HDFS-write failed more than half of the 'retry_limit' setting
update the standby namenode trigger condition
only switch namenode once when StandbyException occurred
to avoid dead loop
rewrite the variables swap code in the ruby-way
improve readability of stanby-namenode support code
add some checks: httpfs and standby_namenode must not be used at same time