Implement virtual DebugSession::killDebuggerNow()

https://commits.kde.org/kdevelop/37ff587ebe2a2c355d71cb583fb5f058c5301ff3
introduced a pure virtual IDebugSession::killDebuggerNow() member
function. DebugSession must override it to become non-abstract and fix
the following compilation error (GCC):
  invalid new-expression of abstract class type ‘XDebug::DebugSession’

DebugSession::killDebuggerNow() mimics the implementation of
DebugSession::stopDebugger().

As far as I can tell, the following events happen synchronously when
Connection::close() is called: &QTcpSocket::disconnected, then
&Connection::closed signals are emitted,
DebugSession::connectionClosed() switches to EndedState, and so
KDevelop::DebugController has a chance to schedule the DebugSession's
deletion.