Modify to use multiple standby_namenodes

[Lion2me]

Signed-off-by: lion2me [email protected]

Hello, I'm Lion2me.

I am using one active namenode and two standby namenodes using the zookeeper.

While using the webhdfs plug-in, I saw that the number of standby names was fixed to one, and I am sending the corrections I made to apply to the current environment as PR.

The major changes are as follows:

Standby nameenode was made to receive multiple values based on the space. In order to switch the nameenode, clients_standby were arranged and modified to access the index. When failover occurred, clients_standby was iterated to find and switch available namesenodes.