buffet: Make TLS shut down gracefully on connection termination

Perform simple unidirectional shutdown of TLS connection when we are
disconnecting from a server. There is no need to wait for the server
response since we are closing the underlying socket anyway.

Added more informational logging for XMPP connection start/teardown.

BUG=brillo:1155
TEST=`FEATURES=test emerge-link libchromeos buffet`

Change-Id: I118e0fbb33073d71583c9874deceac2d38565ec6
Reviewed-on: https://chromium-review.googlesource.com/275028
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>

buffet/notification/xmpp_channel.cc

diff --git a/buffet/notification/xmpp_channel.cc b/buffet/notification/xmpp_channel.cc
index 9bf9463..c651688 100644
--- a/buffet/notification/xmpp_channel.cc
+++ b/buffet/notification/xmpp_channel.cc
@@ -361,6 +361,7 @@
 
 void XmppChannel::Connect(const std::string& host, uint16_t port,
                           const base::Closure& callback) {
+  LOG(INFO) << "Starting XMPP connection to " << host << ":" << port;
   int socket_fd = ConnectSocket(host, port);
   if (socket_fd >= 0) {
     raw_socket_ =