Move orphan processing to ActivateBestChain

This further decouples "main" and "net" processing logic by moving
orphan processing out of the chain-connecting cs_main lock and
into its own cs_main lock, beside all of the other chain callbacks.

Once further decoupling of net and main processing logic occurs,
orphan handing should move to its own lock, out of cs_main.

Note that this will introduce a race if there are any cases where
we assume the orphan map to be consistent with the current chain
tip, however I am confident there is no such case (ATMP will fail
without DoS score in all such cases).