Intern strings in KeyVersion and EncryptedKeyVersion

This is inspired by misha@cloudera.com's work on HDFS-11383.

The key names and key version names are usually the same for a bunch of KeyVersion and EncryptedKeyVersion. We should not create duplicate objects for them.

This is more important to HDFS-10899, where we try to re-encrypt all files' EDEKs in a given EZ. Those EDEKs all has the same key name, and mostly using no more than a couple of key version names.