Hardcoded "_delta_log" dir name

[rambabu-posa]

Any reason to hardcode transaction log's root folder name "_delta_log" in DeltaLog.scala file?

[tdas]

So that given the table directory (from Dataframe API or from location in metastore), it is obvious where the log files are.

[jaceklaskowski]

It could be an option, but what would that flexibility give? It simply easier to assume it's _delta_log and consider it a "magic number" (so external tools know it's a delta table immediately).