Align default port range for default allocation with JVB's default.

[guusdk]

For dynamic port allocation of media streams, the videobridge uses a
default port range of 10001-20000 UDP. To avoid confusion, it would be
good if libjitsi uses the same default. Previously, libjitsi defined
5000-6000 UDP as the default range.

[guusdk]

This change finds its origin in this mailing list thread: http://lists.jitsi.org/pipermail/dev/2018-May/037818.html

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[bgrozev]

I haven't verified, but I think that the jitsi desktop client does not override the default, so this would change its behavior.