This is a revised version of
https://github.com/matrix-org/matrix-js-sdk/pull/1472 which was previously
reverted for causing security prompts to appear on device list sync. In this
version, we only allow private key requests (which are likely to trigger user
dialogs) if we are coming from the bootstrap path.
This allows sessions that have already synced cross-signing public keys but
never got the private keys for some reason to make forward progress when e.g.
the user triggers bootstrap from security settings.
J. Ryan Stinnett
0b063f6b8b