Use state.txt files in ChangesetSource

[jpolchlo]

There is a bug in ChangesetSource here: https://github.com/geotrellis/vectorpipe/blob/v2.2.0/src/main/scala/vectorpipe/sources/ChangesetSource.scala#L79

This line should not access state.yaml directly, but instead should use the provided filename. This prevents us from using the ###.state.txt files in getSequence. Some debugging of code using getSequence may be required at that point.