and rename existing IsConnected const to IsConnectedMark

Decoration is in fact quite a simple process that is applied on entry
to rendering: we take a base renderer, transform it with a decorator,
and then render a report with it. The new render.Decorate() function
does exactly that.

There is one exception. When rendering an individual node, e.g. for
showing its details panel in the UI, we must not lose the node during
decoration. That requires some special logic, which previously resided
in the PreciousNodeRenderer, and now lives in handleNode.

pass render filters and maps by value rather than reference

They are small and don't carry mutable state, so there is no point
passing them by reference.

Parsing optimisations

Pass in a slice on the stack instead of allocating one on the heap:
reduces garbage, hence makes the program run faster

Also apply knowledge that critbitgo will do an append() with one extra
byte, so we do that allocation up-front too.  This is innocuous should
we stop using critbitgo or should its internals change.

These routines are called a lot, so this change reduces garbage collection

Upgrade to React 16

Fix undefined image bug

Use timestamp in URL

Fix incorrect image status text bug

Remove optional dependencies

produce stats as part of rendering

since it was only introduced to because producing stats would hit the
renderer twice, which is no longer the case.

step 2 (and final step) in producing stats as part of Rendering

...so there is no need for a separate Stats method.

step 1: return stats from Render

Optimisation: replace three map-reduces with Renderers

This shows a big improvement in BenchmarkTopologyList

New type joinResult is created to hold the nodes and ID mapping.
The implementation move from host.go to render.go.

Pass 'id' through to the create function and expect that the result Node has that ID.
Extract a function newPseudoNode for common calls.

This is much more efficient as we skip creating then merging all intermediate Nodes

This means we are no longer generating a Counter for the number of
endpoint sub-nodes, but it seems that data was not used.

This is much more efficient as we skip creating then merging all intermediate Nodes

This allows us to avoid creating a host of 'IP' type Nodes then
discarding them after matching; instead we match directly and create
just the result we want.

To time a single topology, for more focused optimisation

Humanize reported durations

remove unused memoisation

Fixes #2923

Avoid object creation when scanning DNS names