conncheck: fix long time to connect if conncheck list gets filled late

When a stream's conncheck list is empty during
priv_conn_check_unfreeze_related() and the check pairs get added only
afterwards, the whole list may remain in frozen state for tens of
seconds until the lists of all other streams are completed.

If a connection check list missed the opportunity to be unfrozen
according to the process in RFC 5245 Section 7.1.3.2.3 because it was
empty at the time, unfreeze it immediately once some check pair gets
added.
12 jobs for checklist-delay in 18 minutes and 10 seconds (queued for 22 seconds)
Status Job ID Name Coverage
  Build
passed #1035100
build autotools

00:01:29

passed #1035101
build meson

00:00:51

passed #1035103
docker windows 1607
build msvc amd64

00:03:26

passed #1035104
docker windows 1607
build msvc x86

00:03:28

passed #1035102
docker windows 1607
build msys2

00:03:34

 
  Test
passed #1037028
distcheck autotools

00:03:01

passed #1035110
doc-and-install meson

00:00:58

passed #1035105
test autotools

00:02:09

passed #1035106
allowed to fail
test autotools valgrind

00:03:09

failed #1035108
allowed to fail
test meson

00:01:26

failed #1035109
allowed to fail
test valgrind meson

00:01:19

failed #1035107
distcheck autotools

00:02:37

 
Name Stage Failure
failed
test meson Test
--- command ---
BUILT_WITH_MESON='1' /builds/xhaakon/libnice/build/tests/nice-test-new-trickle
-------

Full log written to /builds/xhaakon/libnice/build/meson-logs/testlog.txt
Uploading artifacts...
build/meson-logs/: found 4 matching files
Uploading artifacts to coordinator... ok
id=1035108 responseStatus=201 Created token=8s7iXL_1
ERROR: Job failed: exit code 1
failed
test valgrind meson Test
==189== For counts of detected and suppressed errors, rerun with: -v
==189== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 26 from 26)
-------

Full log written to /builds/xhaakon/libnice/build/meson-logs/testlog-valgrind.txt
Uploading artifacts...
build/meson-logs/: found 4 matching files
Uploading artifacts to coordinator... ok
id=1035109 responseStatus=201 Created token=mMMQ4DrF
ERROR: Job failed: exit code 1