Node state is intended to store validation related data that needs to be gated by cs_main. This not the case for the last poll time which is a network only data, so this is causing a cs_main locking for no benefit.
This diff move the last_poll attribute to the CNode itself, and make it atomic. This is not strictly required but it makes it safe to access from other thread should this value be made available from RPC.
There is no change in behavior.
Depends on D14548.