MB-11587 make sure that upr_proxy will be killed even if it gets stuck

...in prim_inet:send

Change-Id: I19339eafca27e5d93eafc61c4103b2c0b42bbe7b
Reviewed-on: http://review.couchbase.org/39003
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>