ChainDB: rate limiting chain selection

While designing for Issue input-output-hk/ouroboros-consensus#387, we supposed scenarios where an adversary could "distribute" their attack amongst more than one of our peers. This would circumvent the as-proposed rate-limiting in the ChainSync client, since that is per-peer.

We considered a few mitigations rapidly, but the first that seemed quite appealing was to introduce a rate-limiting on the chain selection behavior of the ChainDB. Instead of each `addBlock` _always_ triggering a corresponding chain selection, we'd limit ourselves to eg 3 chain switches per second.

This Issue is to flesh out the design and testing approach, carefully consider network effects (eg potentially increased forwarding latency etc), and implement the change.