Fix format of server id & name table in events, targets and items.

Because we are planning to move hostId & hostName in events, targets
and items to servers table, "serverNames" is renamed to "servers"
and add "name" keys to server names.

Example of planning format of events.json:

{
 numberOfEvents: 10,
 events: [...],
 servers: {
   1: {
     name: "Server 1",
     hosts: {
       1: { name: "Host 1" },
       2: { name: "Host 2" },
       ....
     }
   },
   2: {
     name: "Server 2",
     hosts: {
       1: { name: "Host 1" },
       2: { name: "Host 2" } ,
       ....
     }
   }
   ...
 }
}

Host ids & names will be move to it later.