Closes #348 - Clean up ScannerConfiguration and create unit test

Pull request for #348
Currently the code does not handle filtering col vis for scans (because previously could not fetch col vis) : [SnapshotScanner.java line 86][1] [TransactionImpl.java line 208][2]

However the code does handle col vis w/ get at [TransactionImpl.java line 167][3]

[1]: https://github.com/fluo-io/fluo/blob/cccde26316b75545636bb14765b0add3cdaa676d/modules/core/src/main/java/io/fluo/core/impl/SnapshotScanner.java#L86
[2]: https://github.com/fluo-io/fluo/blob/cccde26316b75545636bb14765b0add3cdaa676d/modules/core/src/main/java/io/fluo/core/impl/TransactionImpl.java#L208
[3]: https://github.com/fluo-io/fluo/blob/cccde26316b75545636bb14765b0add3cdaa676d/modules/core/src/main/java/io/fluo/core/impl/TransactionImpl.java#L167