Remove dependency on XDAQ from HcalOnlineDb

A compatibility library has been added called xdaq_compat. It includes a
typedef for replacing log4cplus, and copies the toString function from
toolbox.

Most of the changes introduced in this patch are revertible at compile
time by passing the definition "HAVE_XDAQ" to the compiler.

A summary of the changes follows:

* log4cplus/logger has been replaced with an ostream

* toolbox::toString has been copied into xdaq_compat.h

* xgi/Utils.h was unused and so removed

* Fix inheritance problem from std::exception

  std::exception defines `virtual ~Exception() throw()`, whereas the
  hcal::exception::Exception was trying to overwrite this with `virtual
  ~Exception {}`. This lead to a "looser throw specifier" error. As the
  derived class has no reason to redefine this method, it had been
  removed.

This is a forward port of cms-sw/cmssw#737.