New class QlRequest

Migrate some code that was out of place in QlSession and QlCoreModule to a
new class QlRequest, counterpart to QlResponse.
Prepare for migration from $_SERVER to QlRequest

In order to give up using $_SERVER, QlRequest must be passed around quite
a bit. In this first round, many “request URL” arguments are converted to
QlRequest arguments.
Migrate $_SERVER['REMOTE_REAL_ADDR'] to QlRequest::get_client_local_addr()