Efficient HandleGraph to JSON conversion

[adamnovak]

GraphPeakCaller needs a JSON-format graph input, but vg convert can't make that. Only vg view can make that, and if the input is a generic HandleGraph it gets copied into vg::VG format and then JSON-ified into a string in memory and then printed.

vg/src/subcommand/view_main.cpp

Lines 997 to 998 in c9e7e2b

	} else if (output_type == "json") {
	cout << pb2json(vg_graph->graph) << endl;

We need to be able to efficiently convert graphs to JSON that GraphPeakCaller understands.

GraphPeakCaller doesn't seem to do any path rank reconstruction, so we might also need to make sure we can output paths in their entirety, unlike how we chunk them in the Protobuf representation. (It also seems to want to load all the JSON into memory again.)