Support region specific replication url, replication interval, and sequence numbering from sources like geofabrik.de

I have created a pull request which adds several features and fixes a few bugs:

The replicationUrl parameter written into last.state.txt during import was hard coded as https://planet.openstreetmap.org/replication/minute/ regardless of the data source. I have made it dynamic, based on the replication_url specified in the config.json. This allows region specific imports from a data source like geofabrik.de to output valid last.state.txt parameter values that can now be leveraged by a tool like osmosis to generate region specific changes lists.
The replication_interval parameter value in the config.json is now taken into account when generating the sequence number which is output into the last.state.txt file. Before this fix the sequence number was hard coded to be based on a single minute so when performing the diff command on an hourly or daily changes list the sequence number would be wrong (usually negative).
Fixed bug where the replication_interval parameter in config.json got overwritten by the default flag when no flag is specified (should flags or config.json take priority when both are set?).

Additionally, before the pull request is accepted the following two points should be discussed:

I want to make sure my handling of the BaseOptions.DiffStateBefore parameter in update:state:state.go:FromPbf() function won't break anything since I am now calculating it based on the replication interval. It is not clear to me what the purpose of the BaseOptions.DiffStateBefore parameter is.
What is the purpose of the default 2 hour value for BaseOptions.DiffStateBefore? In the case of a 24H changes list it is causing the sequence to be off by an entire day. Not a problem I suppose but it just feels sloppy. What problem is having this default value trying to solve?

omniscale / imposm3