NR: fix: set PDCP discard timer to infinity

The discard timer is used by the PDCP layer to discard packets that were not transmitted to RLC after some time.

We don't really need it for the moment, maybe never for the default bearer which is a best-effort bearer passing all kind of IP traffic. Maybe for voice over IP kind of traffic it's important, or realtime applications.

I ran an iperf UDP uplink test and saw no error on the PHY layer but still lots of packets lost in the iperf logs, which was very surprising. By analyzing the logs in wireshark I saw that, with the current scheduler, an IP packet sent by the UE took 40ms to be sent piece by piece to the gNB, and then the next uplink IP packet was never sent at all because the discard timer was expired, leading to the high count of packets lost in the iperf logs.

So let's put infinity for the moment for the discard timer. From my understanding of the moment it's not a problem. If for some reason we should set it to some non-infinity value, then we can change later.