Make prepareToEncrypt cancellable.
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 https://github.com/matrix-org/matrix-js-sdk/issues/1255
Closes #1255
Signed-off-by: Clark Fischer <clark.fischer@gmail.com>
