Clean up/improve e2e logging

In an attempt to make the rageshake logs a bit more useful, try to make the
logging a bit saner. Firstly, make sure we log every decryption failure, and
log it exactly once, rather than in several places. Also record when we receive
megolm keys. Also add some more explicit logging in the sync loop.

src/sync.js

@@ -360,9 +360,6 @@ SyncApi.prototype.getSyncState = function() {
  * Main entry point
  */
 SyncApi.prototype.sync = function() {
-    debuglog("SyncApi.sync: starting with sync token " +
-             this.client.store.getSyncToken());
-
     const client = this.client;
     const self = this;
 
@@ -511,11 +508,14 @@ SyncApi.prototype._sync = function(syncOptions) {
     // normal timeout= plus buffer time
     const clientSideTimeoutMs = this.opts.pollTimeout + BUFFER_PERIOD_MS;
 
+    debuglog('Starting sync since=' + syncToken);
     this._currentSyncRequest = client._http.authedRequest(
         undefined, "GET", "/sync", qps, undefined, clientSideTimeoutMs,
     );
 
     this._currentSyncRequest.done(function(data) {
+        debuglog('Completed sync, next_batch=' + data.next_batch);
+
         // set the sync token NOW *before* processing the events. We do this so
         // if something barfs on an event we can skip it rather than constantly
         // polling with the same token.
@@ -666,8 +666,10 @@ SyncApi.prototype._processSyncResponse = function(syncToken, data) {
                         toDeviceEvent.getType() == "m.room.message" &&
                             content.msgtype == "m.bad.encrypted"
                     ) {
-                        console.warn(
-                            "Unable to decrypt to-device event: " + content.body,
+                        // the mapper already logged a warning.
+                        console.log(
+                            'Ignoring undecryptable to-device event from ' +
+                                toDeviceEvent.getSender(),
                         );
                         return;
                     }