Replace Session_object::warning() and friends by just print'able Session_object

[chelmuth]

If warning() is used in a static class function of a class derived from Session_object the compiler nags like follows, which can be addressed by using Genode::warning() but is unexpected at least.

error: cannot call member function ‘void Genode::Session_object< <template-parameter-1-1>, <template-parameter-1-2> >::error(auto:176&& ...) const

It may be a better approach to amend Session_object with a print() method such that a derived class can use warning(*this, " deserves a warning").