QUIC: improved setting the lost timer.

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