Fix ranch_server:set_connections_sup/2 race conditions

A ranch_conns_sup could be (re)started and call
ranch_server:set_connections_sup/2 before ranch_server has handled the
predecessor's exit. This would cause the ranch_server to crash because
ets:insert_new/2 would return false.

This change allows ranch_server to handle this case by crashing the
calling process instead of itself.
Merge branch 'fix-server-racecon' of git://github.com/fishcakez/ranch