use gen_udp instead of gen_tcp for address changed alert testing
use gen_udp instead of gen_tcp for address changed alert testing

Hopefully it'll fix our rare false positives from this code caused by
some system limits.

gen_tcp:listen is doing full bind + listen and apparently is not
passing SO_REUSEADDR. That's our theory at least.

With UDP this is not needed and is much lighter. I.e. it's not
creating those TIME_WAIT tcp sockets.

Change-Id: Iab27376efdc6a5196e95b653aa7b12ee07213f9e
Reviewed-on: http://review.couchbase.org/15472
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,mschoch/ns_server,daverigby/ns_server,daverigby/ns_server,membase/ns_server,membase/ns_server,pavel-blagodov/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,vzasade/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,pavel-blagodov/ns_server,ceejatec/ns_server,membase/ns_server,daverigby/ns_server,membase/ns_server,ceejatec/ns_server,nimishzynga/ns_server,mschoch/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,membase/ns_server,ceejatec/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,daverigby/ns_server,vzasade/ns_server