Add API for retrieving graph state by datetime

[bzurkowski]

Add API for retrieving graph state by datetime. For the use in openrca/orca-ui#26.

The UI sends a request to Graph API with an additional datetime parameter in request path. The API should query the graph with the parameter and calculate graph state based on timestamp fields in node and link objects.

Check Skydive time filter implementation. In our case, the implementation will be simpler since we filter by single point in time instead of by a time slice.