Nexus Boot: Start quiescing if target bp `nexus_generation` is greater than our `nexus_generation`

[smklein]

[davepacheco]

I assume this refers to this part of RFD 588:

The Nexus startup path needs to be changed to load the current target blueprint and determine if quiescing is needed before allowing any sagas to be created. The goal is to prevent new sagas from being created if we’re quiesced. It’s not yet clear if it makes more sense to try to do this synchronously during startup (so that we don’t try to run background tasks or open the external API listening socket before we’ve determined if we need to quiesce) or if we want to do this asynchronously and simply fail attempts to create new sagas if we haven’t figured this out yet.

There's an extra bit not mentioned in the title above which is that sagas must not be allowed until we've loaded the target blueprint and confirmed we're not quiescing.

[davepacheco]

Fixed by #8875.