XEP-0198: Check whether routed packets are stanzas

Only stanzas are subject to stream management, so when XEP-0198 support is enabled, we must distinguish them from non-stanza elements.  This commit adds a `send_packet/2` function that can be used in place of `send_stanza/2` or `send_element/2` whenever a packet is delivered that might or might not be a stanza.