Speed up ingestion of CT log data

[jschanck]

CT logs rate-limit our get-entries requests, and this limits how quickly we can "reboot" CRLite when we make a change that requires us to re-ingest CT log data. This issue tracks our progress on speeding up the ingestion of CT log data.

We're currently planning three changes:

1. Use consistency proofs to verify that entries we downloaded (potentially from a third party) match the entries in a CT log.
2. Download entries from CT logs in most-recent-first order. The idea here is that we can get entries {0, 1, ..., k} from a third party, so we should spend our get-entries requests on new entries.
3. Check for log entries in a local disk cache before making a get-entries request.