offload creating an olm session to the olm worker

so IE11 doesn't lock up when you start typing

src/matrix/e2ee/Account.js

@@ -149,10 +149,14 @@ export class Account {
         }
     }
 
-    createOutboundOlmSession(theirIdentityKey, theirOneTimeKey) {
+    async createOutboundOlmSession(theirIdentityKey, theirOneTimeKey) {
         const newSession = new this._olm.Session();
         try {
-            newSession.create_outbound(this._account, theirIdentityKey, theirOneTimeKey);
+            if (this._olmWorker) {
+                await this._olmWorker.createOutboundOlmSession(this._account, newSession, theirIdentityKey, theirOneTimeKey);
+            } else {
+                newSession.create_outbound(this._account, theirIdentityKey, theirOneTimeKey);
+            }
             return newSession;
         } catch (err) {
             newSession.free();

src/matrix/e2ee/OlmWorker.js

@@ -37,6 +37,12 @@ export class OlmWorker {
         account.unpickle("", pickle);
     }
 
+    async createOutboundSession(account, newSession, theirIdentityKey, theirOneTimeKey) {
+        const accountPickle = account.pickle("");
+        const sessionPickle = await this._workerPool.send({type: "olm_create_outbound", accountPickle, theirIdentityKey, theirOneTimeKey}).response();
+        newSession.unpickle("", sessionPickle);
+    }
+
     dispose() {
         this._workerPool.dispose();
     }

src/matrix/e2ee/olm/Encryption.js

@@ -154,7 +154,7 @@ export class Encryption {
         try {
             for (const target of newEncryptionTargets) {
                 const {device, oneTimeKey} = target;
-                target.session = this._account.createOutboundOlmSession(device.curve25519Key, oneTimeKey);
+                target.session = await this._account.createOutboundOlmSession(device.curve25519Key, oneTimeKey);
             }
             this._storeSessions(newEncryptionTargets, timestamp);
         } catch (err) {

src/platform/web/worker/main.js

@@ -136,6 +136,18 @@ class MessageHandler {
             account.generate_one_time_keys(otkAmount);
             this._checkRandomValuesUsed();
             return account.pickle("");
+    _olmCreateOutbound(accountPickle, theirIdentityKey, theirOneTimeKey) {
+        return this._toMessage(() => {
+            const account = new this._olm.Account();
+            const newSession = new this._olm.Session();
+            try {
+                account.unpickle("", accountPickle);
+                newSession.create_outbound(account, newSession, theirIdentityKey, theirOneTimeKey);
+                return newSession.pickle("");
+            } finally {
+                account.free();
+                newSession.free();
+            }
         });
     }
 
@@ -149,6 +161,8 @@ class MessageHandler {
             this._sendReply(message, this._megolmDecrypt(message.sessionKey, message.ciphertext));
         } else if (type === "olm_create_account_otks") {
             this._sendReply(message, this._olmCreateAccountAndOTKs(message.randomValues, message.otkAmount));
+        } else if (type === "olm_create_outbound") {
+            this._sendReply(message, this._olmCreateOutbound(message.accountPickle, message.theirIdentityKey, message.theirOneTimeKey));
         }
     }
 }