Fix statics in CondFormats/EcalObjects.

- Make dummy elements thread_local. More correct solution is probably simply to
  throw.
- In case of the const API, simply change it to return by value, since Item is
  in any case basically a int.
- Add default constructor for a few classes.
Merge pull request #1417 from ktf/sr-CondFormats-EcalObjects

Multithreading fixes -- Make dummy elements thread_local.