Upload only new device signature of master key
Upload only new device signature of master key

This changes bootstrap to only upload the new device signature of the master
key. We were previously _adding_ the new signature, but then uploading both old
and new device key signatures of the master key.

This was particularly bad when re-uploading signatures from deleted devices, as
that would cause the homeserver to reject the entire upload.

Fixes https://github.com/vector-im/riot-web/issues/12752

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