Improve sg-replicate logging integration

Need to address two issues related to logging generated by sg-replicate:

1. We don't have a way to enable debug logging via Sync Gateway's config.  At minimum we want the ability to call EnableSgReplicateLogging if the "Replicate" log key is enabled with log level debug or trace.
2. Error/warn logging within sg-replicate isn't sent to Sync Gateway's logs. 

The second issue may require enhancing sg-replicate to accept Sync Gateway's logging interface, instead of using clog.  Needs further analysis to identify the right approach.