Renamed 'open' fields on BigdataSail and BigdataSailConnection as openSail and openConn to help identify incorrect inheritance problems.
Renamed assertOpen() methods as assertOpenConn() and assertOpenSail().
Renamed assertWritable() as assertWritableConn()
Modified toString() on the connection classes to show whether the connection is open.
Made the anonymous BigdataSailConnection classes into named inner classes in an attempt to prevent future ambiguity in the inheritance patterns.
Note: It would be far better if BigdataSailConnection were a static class since that would remove the opportunity entirely for incorrect inheritance patterns.