During partitions which cause unbounded DB lag increases:
a) Have WANObjectCache automatically use small TTLs if slave lag gets too close to the HOLDOFF threshold
This is useful for a full-network partition scenario between DCs. We could have a DC-local cached value trigger this. A one-thread-at-time deferred update can contact the master and wait for slaves to make sure things are OK. If it fails, the WAN cache will know (from reading the key) that something is wrong and lower all set() TTLs to stop stale data from getting stuck for the whole set() TTL (that could be days, weeks...).
b) Likewise with CDN expiry headers
c) Trigger wfReadOnly() in the slave DC to discourage form submission (if we use pt-heartbeat this will happen for free).