Make a ZeroOrMoreTQLayer modifier which is limited and continuable

[mwatts15]

The ZeroOrMoreTQLayer grabs all relationships along a given predicate in the given graph. As we work with larger graphs we may want to limit how far we go to a smaller radius than the whole available graph. In the case that we hit the limit before running out of search space, we can ask the caller whether to proceed, cancel, or continue depending on the needs of the application. Making the layer 'continuable' means that in various ways we 'prime the pump' using already-computed information (e.g., the border of the limited traversal) to avoid re-doing work.