Upload cross-signing key signatures in the background
Upload cross-signing key signatures in the background

At the moment, uploading cross-signing key signatures is a slow process that can
potentially take many minutes (!) for large accounts / slow servers. This
changes to do the bootstrapping related versions of this in the background.

Note that key signature uploads for interactive flows like verification are
still blocking for now.

Fixes https://github.com/vector-im/riot-web/issues/12223

matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk