Rework the pre-key management

[rubdos]

Our pre-key management has some issues, since a while.

1. When the server pre keys drop below 10, we upload 100 new keys. The x<10 previous ones stay in storage, but get removed from the server. Post-quantum E2E sessions #234 (comment)
2. We don't implement the last-resort post-quantum key. Post-quantum E2E sessions #234 (comment)

Signal-upstream solves this by syncing the local pre-key state to the server, instead of trying to update the server and storage synchronously. We should revamp the pre-key management logic to reflect the same idea.