Allow a limit to be specified for the number of open files used by a partitioned source
Use option instead of default value.  This change also means that users can specify a threshold less than or equal to zero.  There doesn't seem to be any error checking in Cascading that forbids this, but the end result is that it's as if they passed in a threshold of 1.
Previous commit was incomplete and caused build to fail.  This will fix it...
Merge pull request #1078 from matt-martin/PartitionSourcesWithFileLimit

Allow PartitionSource to limit the number of open files