QUIC: improved setting the lost timer. Setting the timer is brought into compliance with quic-recovery-34. Now it's set from a single function ngx_quic_set_lost_timer() that takes into account both loss detection and PTO. The following issues are fixed with this change: - when in loss detection mode, discarding a context could turn off the timer forever after switching to the PTO mode - when in loss detection mode, sending a packet resulted in rescheduling the timer as if it's always in the PTO mode