Make sure that proxy server doesn't try to open connection to the
target multiple times. When client is uploading huge amount of data
it might happen that proxy_rx_callback is called again before
connection to the target is established.
Type: fix
Change-Id: I4d272d37bc545f6f8790a115c8d26bcf3ea84f22
Signed-off-by: Matus Fabian <matfabia@cisco.com>