Revert a4233edba (No need to &= ~KEEP_ALIVE...)

A request might change its Connection status from keep-alive to close,
and not resetting this flag will maintain it as a keep-alive connection.
Cosmetic cleanups in _read_request()
Do not reuse request buffer for other purposes
Get rid of `buffer` field in `lwan_request_t`
Add some crude logging stuff

This is only compiled in debug mode. Proper logging is underway.