Reduce blocking while pre-fetching Megolm keys
Currently, calling `Client#prepareToEncrypt` in a megolm room has the potential
to block for multiple seconds while it crunches numbers.
Sleeping for 0 seconds (approximating `setImmediate`) allows the engine to
process other events, updates, or re-renders in between checks.
See
- https://github.com/vector-im/element-web/issues/21612
- https://github.com/vector-im/element-web/issues/11836
Signed-off-by: Clark Fischer <clark.fischer@gmail.com>
