Replace `trillian_log_server.log_id_ranges` flag with a config file (#742)

* Replace trillian_log_server.log_id_ranges flag with a config file

This will make it easier to specify mulitple shards, along with associated tree IDs and lengths.
Each shard may eventually have its own signer/public key as well, so it'll be easier to pass those in through
a config file rather than through CLI flags.

Signed-off-by: Priya Wadhwa <priya@chainguard.dev>

* Add active tree ID to ranges

Signed-off-by: Priya Wadhwa <priya@chainguard.dev>