support switching startEvent while waiting for .accept on initiator side

if we get a .start event from the other party and we've also sent one,
the .start event with the sender that is first in sorting order should
be taken, and the other one ignored.

At the point where we will receive it, the verifier has already
been returned from beginKeyVerification, so we'll need to switch
start event internally, and retry the verification, now on the
receiver (sending .accept) side instead of initiator side
(sending .start).

src/crypto/verification/Base.js

@@ -27,6 +27,13 @@ import {newTimeoutError} from "./Error";
 
 const timeoutException = new Error("Verification timed out");
 
+export class SwitchStartEventError extends Error {
+    constructor(startEvent) {
+        super();
+        this.startEvent = startEvent;
+    }
+}
+
 export class VerificationBase extends EventEmitter {
     /**
      * Base class for verification methods.
@@ -69,6 +76,19 @@ export class VerificationBase extends EventEmitter {
         this._transactionTimeoutTimer = null;
     }
 
+    get initiatedByMe() {
+        // if there is no start event yet,
+        // we probably want to send it,
+        // which happens if we initiate
+        if (!this.startEvent) {
+            return true;
+        }
+        const sender = this.startEvent.getSender();
+        const content = this.startEvent.getContent();
+        return sender === this._baseApis.getUserId() &&
+            content.from_device === this._baseApis.getDeviceId();
+    }
+
     _resetTimer() {
         logger.info("Refreshing/starting the verification transaction timeout timer");
         if (this._transactionTimeoutTimer !== null) {
@@ -104,6 +124,22 @@ export class VerificationBase extends EventEmitter {
         });
     }
 
+    canSwitchStartEvent() {
+        return false;
+    }
+
+    switchStartEvent(event) {
+        if (this.canSwitchStartEvent(event)) {
+            if (this._rejectEvent) {
+                const reject = this._rejectEvent;
+                this._rejectEvent = undefined;
+                reject(new SwitchStartEventError(event));
+            } else {
+                this.startEvent = event;
+            }
+        }
+    }
+
     handleEvent(e) {
         if (this._done) {
             return;