Historic room key bundles are uploaded as an encrypted file to the media
repo and the key to decrypt the file is sent as a to-device message to
the recipient device.
In the nominal case, the invite and this to-device message should arrive
at the same time and accepting the invite would download and import the
bundle.
If the to-device message arrives after the invite has already been
accepted we would never download and import the bundle.
To mitigate this problem, this patch introduces a task that listens for
bundles that arrive. If the bundle is for a room that we have joined we
will consider importing the bundle.
Damir Jelić
2bbf6fc711