make Report.Toplogy(name) fast
make Report.Toplogy(name) fast

Previously this was buidling a fresh map of all topologies, just so it
could look up the one given as an argument.

Node summarisation (via detailed.Summaries) in particular was badly
affected by that.

kinvolk/scope,alban/scope,kinvolk/scope,alban/scope,weaveworks/scope,alban/scope,weaveworks/scope,alban/scope,kinvolk/scope,alban/scope,weaveworks/scope,kinvolk/scope,weaveworks/scope,kinvolk/scope,kinvolk/scope,alban/scope,weaveworks/scope,weaveworks/scope