tokio-xmpp: Send the initial stream:stream in a single packet

Prosody would let the stream opening timeout if it receives it in more
than one TCP packet, which seems like a bug.

The behaviour was highly non-deterministic, a release build would almost
always (but not always) succeed to connect to a localhost Prosody, a
debug build would almost always (but not always) fail, and
RUST_LOG=trace would make any build fail to connect, as would strace.

My theory is that successful runs rely on the kernel merging all five
TCP packets in a single one, and any tracing would add sufficient delay
in-between to let it send every packet on its own.

tokio-xmpp/ChangeLog

@@ -16,6 +16,8 @@ Version NEXT:
         `tokio_xmpp::connect::DnsConfig`
     * Fixed:
       - Ignore missing "version" stream attribute for 0114 components.
+      - Always send the stream:stream opening in one TCP packet, to workaround
+        issue #1995 in Prosody. (!680)
       - Gate `AsRawFd` behind `ktls` feature to make Windows build work again.
       - Implicitly use the `Client`'s bound JID on empty `from` and `to` in
         tokio-xmpp's `IqResponseTracker`.