fixup lists formatting in README
add Ubuntu-specific notes
fix CactiGraph SQL indexing

The original support for Cacti graphs (commit bfbdd78) allowed for a
graph to be linked at most once due to the unique constraint on
CactiGraph.graph_id. The multi-server support (commit 14a9812) changed
that to be at most once per (server_id, graph_id) pair.

This change fixes the unique constraint to be once per (object_id,
server_id, graph_id) triplet, which seems to have most sense.