Add support for SmartIF templated with a concrete type

This is another attempt at #344 (closed).

Instead of addressing the specific case of ServiceHandle, I operated at a lower level, so that SmartIF can work either via queryInterface or via dynamic_cast. Ideally, we should replace every direct use of queryInterface with IInterface::cast and ensure that all custom implementations of queryInterface are replaced with custom implementations of IInterface::i_cast.

Because I changed ServiceHandle to rely on SmartIF, I broke ServiceHandle<const T>, so I decided to fix #20 (closed) here.

Changes:

• improved and extended IInterface::cast to work either via i_cast or dynamic_cast
  • with a fallback on queryInterface while we replace queryInterface with i_cast
  • the fallback required a hepler method to decrease the refcount without deleting the object
• modified SmartIF to leverage on IInterface::cast
• modified ServiceHandle to rely on SmartIF
• modified Gaudi::Cast to use IInterface::cast (for consistency)
• added unit tests for SmartIF
• fixed support for SmartIF<const T> (#20 (closed))
• changed the default implementation of IInterface::release to avoid possible race conditions

As a bonus, I made SmartIF constructible from unique_ptr, with the idea of later on deprecate creation from bare pointers.

Closes #344 (closed), #20 (closed)