StatCalc creation is moved to MultiUrlRequestInfo. The summary output
is done only when PREF_QUIET is false and it is always to
global::cout(), so remove summaryOut_ and just use global::cout() in
that case. Also use std::unique_ptr for statCalc_ in DownloadEngine.
sessionConfigSetKeepRunning() was removed and SessionConfig has
keepRunning member instead. Signal handlers are now all prepared in
MultiUrlRequestGroup object. Setting SessionConfig.useSignalHandler to
false will avoid signal handler setup.