turn: Allow lifetimes under 60s

Don't set a timeout to the very far future if the lifetime sent  by the
server is under 60s. Instead of doing "lifetime - 60", switch to lifetime/2.
6 jobs for <span class="ref-name">64-short-turn-lifetime</span> in 5 minutes and 35 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #92598
build autotools

00:01:32

passed #92599
build meson

00:00:37

 
  Test
passed #92601
distcheck autotools

00:02:09

passed #92603
doc-and-install meson

00:00:37

passed #92600
test autotools

00:02:00

passed #92602
allowed to fail
test meson

00:00:38