use a separate object for each encrypted content

so that we don't duplicate the ciphertext for everyone

src/crypto/algorithms/megolm.js

@@ -460,15 +460,15 @@ MegolmEncryption.prototype._splitBlockedDevices = function(devicesByUser) {
 MegolmEncryption.prototype._encryptAndSendKeysToDevices = function(
     session, chainIndex, userDeviceMap, payload,
 ) {
+    const contentMap = {};
+
+    const promises = [];
+    for (let i = 0; i < userDeviceMap.length; i++) {
         const encryptedContent = {
             algorithm: olmlib.OLM_ALGORITHM,
             sender_key: this._olmDevice.deviceCurve25519Key,
             ciphertext: {},
         };
-    const contentMap = {};
-
-    const promises = [];
-    for (let i = 0; i < userDeviceMap.length; i++) {
         const val = userDeviceMap[i];
         const userId = val.userId;
         const deviceInfo = val.deviceInfo;