Improve connection handling

1. Previously unhandled connections will stay in pending state. It won't
be closed until timeout happened. This may lead to wasting system
resources. Now the (over-limit) connection is actively rejected.

2. When out-of-memory occurs here, reject the new connection instead of
throwing exception and crash.

3. Also clean up some unused bits.

PR #20961.

src/base/bittorrent/bencoderesumedatastorage.cpp

@@ -40,7 +40,6 @@
 #include <QRegularExpression>
 #include <QThread>
 
-#include "base/algorithm.h"
 #include "base/exceptions.h"
 #include "base/global.h"
 #include "base/logger.h"