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.
* Add more checks and also more strict checks for invalid conditions
* Add http version field
* Raise max request size to 64 MB
* Add author in license
* Use Qt5 new connect syntax
