Store outbound Megolm sessions for faster encryption after a restart/refresh

### Your use case

#### What would you like to do?
I'd like to send my messages faster after updating Element.

#### Why would you like to it?
So I can send messages in large rooms of colleagues without having to wait minutes for encryption.

#### How would you like to achieve it?
Element doesn't appear to be storing outbound Megolm sessions, but if it did then it would mean getting back into the swing of things faster. There's still a risk that the user would run into issues when the sessions naturally rotate, but that would be a separate issue.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_