Cleanup findbug warnings in branch-3.4: Malicious code vulnerability Warnings

MS	org.apache.zookeeper.Environment.JAAS_CONF_KEY isn't final but should be
Bug type MS_SHOULD_BE_FINAL (click for details) 
In class org.apache.zookeeper.Environment
Field org.apache.zookeeper.Environment.JAAS_CONF_KEY
At Environment.java:[line 34]

MS	org.apache.zookeeper.server.ServerCnxn.cmd2String is a mutable collection which should be package protected
Bug type MS_MUTABLE_COLLECTION_PKGPROTECT (click for details) 
In class org.apache.zookeeper.server.ServerCnxn
Field org.apache.zookeeper.server.ServerCnxn.cmd2String
At ServerCnxn.java:[line 230]

MS	org.apache.zookeeper.ZooDefs$Ids.OPEN_ACL_UNSAFE is a mutable collection
Bug type MS_MUTABLE_COLLECTION (click for details) 
In class org.apache.zookeeper.ZooDefs$Ids
Field org.apache.zookeeper.ZooDefs$Ids.OPEN_ACL_UNSAFE
At ZooDefs.java:[line 100]

MS	org.apache.zookeeper.ZooKeeperMain.commandMap is a mutable collection which should be package protected
Bug type MS_MUTABLE_COLLECTION_PKGPROTECT (click for details) 
In class org.apache.zookeeper.ZooKeeperMain
Field org.apache.zookeeper.ZooKeeperMain.commandMap
At ZooKeeperMain.java:[line 53]