xAODCaloEventCnv: Making setSignalState non-const.

Change the ISignalState::setSignalState and resetSignalState methods
to be (properly!) non-const.  SignalStateHelper now takes a non-const
pointer to the object.  (Temporarily?) introduce SignalStateConstHelper
that takes a const pointer and does a const_cast on it; this is marked
as not thread-safe.

Remove now-unused SignalStateCollHelper.

Corresponding changes downstream.