Return virtual index when creating and getting a log entry (#725)

* Return virtual index when creating and getting a log entry

Use the virtual index when signing an entry on creation, and return that to the end user.
There shouldn't be any observable difference here at the moment, until we actually shard the log.

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

* Remove pointer to logRanges so value can't be modified

Also make all fields private and only accessible via funcition calls

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

* Fix virtual log index bug when getting indicies in inactive shards

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