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 master in 4 minutes and 20 seconds (queued for 3 minutes and 2 seconds)
Status Job ID Name Coverage
  Build
passed #92604
build autotools

00:01:09

passed #92605
build meson

00:00:54

 
  Test
passed #92607
distcheck autotools

00:02:51

passed #92609
doc-and-install meson

00:00:57

passed #92606
test autotools

00:01:34

passed #92608
allowed to fail
test meson

00:00:57