crimson/net/Socket: do not reset FixedCPUServerSocket::shutdown_gate
the copy constructor of seastar::gate is deleted explicitly. so we
cannot reset FixedCPUServerSocket::shutdown_gate by assigning a new
seastar::gate to it.
since we don't reuse a FixedCPUServerSocket after calling
FixedCPUServerSocket::destroy(), it's safe to leave a closed gate after
calling FixedCPUServerSocket::reset()