Adds an async/promise-based version of `setImmediate`. Note that, despite being
poorly adopted, `setImmediate` is polyfilled, and should be more performant
than `sleep(0)`.

Signed-off-by: Clark Fischer <clark.fischer@gmail.com>

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
- element-hq/element-web#21612
- element-hq/element-web#11836

Signed-off-by: Clark Fischer <clark.fischer@gmail.com>

NOTE: This commit introduces a backwards-compatible API change.

Adds the ability to cancel `MegolmEncryption#prepareToEncrypt` by returning
a cancellation function. The bulk of the processing happens in
`getDevicesInRoom`, which now accepts a 'getter' that allows the caller to
indicate cancellation.

See matrix-org#1255
Closes matrix-org#1255

Signed-off-by: Clark Fischer <clark.fischer@gmail.com>