unset is_sending_ during tce restart

AOS-202387

During tce restart() queue_ will be drained, messages printed like

tce::restart: dropping message: remote:10.6.0.3:30510 (30fd): [0402.0001.018e] size: 23

and is_sending_ doesn't get unset. Later in connect_cbk there's nothing
to send, nobody will clear the flag, and the train logic remains blocked
from sending. This needs to be cleared during socket restart

implementation/endpoints/src/tcp_client_endpoint_impl.cpp

@@ -125,6 +125,7 @@ void tcp_client_endpoint_impl::restart(bool _force) {
             }
             self->queue_.clear();
             self->queue_size_ = 0;
+            self->is_sending_ = false;
         }
         VSOMEIP_WARNING << "tce::restart: local: " << address_port_local
                 << " remote: " << self->get_address_port_remote();