use ldes metadata to determine version path and timestamp path

[nvdk]

From the spec:

The ldes:EventStream instance MAY have these properties:

    ldes:timestampPath indicating how you can understand using a timestamp (xsd:dateTime) a member precedes another member in the LDES

 ldes:versionOfPath indicating the non-version object (see example bellow).

We should probably use that info if available and fallback to something otherwise (note that #14 makes the defaults configurable)