R1:ebff07d01983

Automatically sever databases after prolonged unreachability

Summary:
Ref T4571. When a database goes down briefly, we fall back to replicas.

However, this fallback is slow (not good for users) and keeps sending a lot of traffic to the master (might be bad if the root cause is load-related).

Keep track of recent connections and fully degrade into "severed" mode if we see a sequence of failures over a reasonable period of time. In this mode, we send much less traffic to the master (faster for users; less…