Log envelopes and session end events

Help debug bad session errors by logging some envelope info about the
message we are about to decrypt. With this, if there is a decryption
error (e.g., bad mac or no session) it is clear from the logs what
number and device message sent the bad message.

Also log when we send and receive end session messages and when we close
sessions for certain devices.

// FREEBIE

libtextsecure/sendmessage.js

@@ -153,12 +153,14 @@ MessageSender.prototype = {
     },
 
     closeSession: function(number, timestamp) {
+        console.log('sending end session');
         var proto = new textsecure.protobuf.DataMessage();
         proto.body = "TERMINATE";
         proto.flags = textsecure.protobuf.DataMessage.Flags.END_SESSION;
         return this.sendIndividualProto(number, proto, timestamp).then(function(res) {
             return textsecure.storage.devices.getDeviceObjectsForNumber(number).then(function(devices) {
                 return Promise.all(devices.map(function(device) {
+                    console.log('closing session for', device.encodedNumber);
                     return textsecure.protocol_wrapper.closeOpenSessionForDevice(device.encodedNumber);
                 })).then(function() {
                     return res;