Add topology.ReplaceNode() for efficiency

[bboreham]

In some places AddNode() was called after adding to an existing node, in which case the Merge() is just a waste of time.

Spotted in a memory profile of the probe: about 14% of all memory allocated is in Merge():

----------------------------------------------------------+-------------
      flat  flat%   sum%        cum   cum%   calls calls% + context 	 	 
----------------------------------------------------------+-------------
                                        16356.22MB 41.27% |   github.com/weaveworks/scope/probe.topologyTagger.Tag.func1 /go/src/github.com/weaveworks/scope/probe/topology_tagger.go
                                        12215.04MB 30.82% |   github.com/weaveworks/scope/probe/host.Tagger.Tag /go/src/github.com/weaveworks/scope/probe/host/tagger.go
                                         4821.62MB 12.17% |   github.com/weaveworks/scope/probe/endpoint.(*connectionTracker).addConnection /go/src/github.com/weaveworks/scope/probe/endpoint/connection_tracker.go
                                         2768.72MB  6.99% |   github.com/weaveworks/scope/probe/docker.(*Tagger).tag /go/src/github.com/weaveworks/scope/probe/docker/tagger.go
                                         1511.73MB  3.81% |   github.com/weaveworks/scope/probe/process.(*Reporter).processTopology.func1 /go/src/github.com/weaveworks/scope/probe/process/reporter.go
                                          557.68MB  1.41% |   github.com/weaveworks/scope/probe/endpoint.natMapper.applyNAT.func1 /go/src/github.com/weaveworks/scope/probe/endpoint/nat.go
                                          400.83MB  1.01% |   github.com/weaveworks/scope/probe/docker.(*Reporter).containerTopology /go/src/github.com/weaveworks/scope/probe/docker/reporter.go
                                          375.10MB  0.95% |   github.com/weaveworks/scope/probe/docker.(*Reporter).containerImageTopology.func1 /go/src/github.com/weaveworks/scope/probe/docker/reporter.go
...
         0     0%     0% 39630.96MB 17.84%                | github.com/weaveworks/scope/report.Topology.AddNode /go/src/github.com/weaveworks/scope/report/topology.go
                                        34675.80MB 87.50% |   github.com/weaveworks/scope/report.Node.Merge /go/src/github.com/weaveworks/scope/report/node.go
                                         4955.16MB 12.50% |   runtime.mapassign /usr/local/go/src/runtime/hashmap.go
----------------------------------------------------------+-------------

The three call sites in this PR are number 1, 2 and 4 in the above profile; number 3 is not directly replacing an existing node so left as-is.

[rade]

In some places AddNode() was called after adding to an existing node, in which case the Merge() is just a waste of time.

That is only correct if all information in the existing node that needs preserving is also present in the new node. It is not at all obvious whether that is the case for the call sites changed in this PR.

[bboreham]

I changed back one case where there is a curious variable shadowing to confuse the unwary.

[rade]

LGTM, with one minor improvement suggestion.

[rade]

Pls squash before merging.