Renamed OpenCluster/OpenDatabase methods to be in line with the other bindings

- All Fdb.OpenAsync(....) methods will return an FdbDatabase instance.
- All Fdb.CreateClusterAsync(...) methods will return an FdbCluster instance.
- OpenLocalXYZ methods are removed, because they do not open the db on the *local* machine, but uses the *default* cluster file that can point anywhere.
- Most users will probably use Fdb.OpenAsync() that opens the "DB" db using the default cluster file.