Fix timeline jumpiness by setting correct txnId

[germain-gg]

Fixes element-hq/element-web#16914

Regression introduced by #1655

I incorrectly tried to remove pending events in updatePendingEvent when this reconciliation is the responsability of that transaction ID. This PR sets that txnId attribute properly and remove the need to explicitely call removePendingEvent as the event is now going through the usual timeline flow

@t3chguy raised an interesting comment in my previous pull request that is adressed here. The txnId is now persisted in storage and re-used at a later stage to ensure idempotency

[germain-gg]

@t3chguy / @jryans thank you for your review earlier.

I have undone my changes to event.toJSON to avoid changing the "View Source" payload.
In addition to that the messages persisted to localStorage respect the "encryption" setting set by the room they were sent in

Using the spread operator I can persist all the enumerable properties on the event and add the txnId. Failing to do that rings all alarm bells about a potential replay attack

[jryans]

Perhaps sprinkle in some comments on encryption expectations, but otherwise seems good to me. 😄