Another implicit invariant in the data model is that edges are always of the
form (local -> remote). That is, the source of an edge must always be a node
that originates from within Scope's domain of visibility. This was evident by
the presence of ingress and egress fields in edge/aggregate metadata.

When building the sniffer, I accidentally and incorrectly violated this
invariant, by constructing distinct edges for (local -> remote) and (remote ->
local), and collapsing ingress and egress byte counts to a single scalar. I
experienced a variety of subtle undefined behavior as a result. See #339.

This change reverts to the old, correct methodology. Consequently the sniffer
needs to be able to find out which side of the sniffed packet is local v.
remote, and to do that it needs access to local networks. I moved the
discovery from the probe/host package into probe/main.go.

As part of that work I discovered that package report also maintains its own,
independent "cache" of local networks. Except it contains only the (optional)
Docker bridge network, if it's been populated by the probe, and it's only used
by the report.Make{Endpoint,Address}NodeID constructors to scope local
addresses. Normally, scoping happens during rendering, and only for pseudo
nodes -- see current LeafMap Render localNetworks. This is pretty convoluted
and should be either be made consistent or heavily commented.

NewNodeMetadata -> MakeNodeMetadata. It doesn't return a pointer, so
Make is more idiomatic.

Invoke MakeNodeMetadata when necessary. The zero value for a
NodeMetadata is no longer valid.

Split MakeNodeMetadata to two constructors. MakeNodeMetadata when you
don't have anything to prepopulate; MakeNodeMetadataWith when you do.

Also, a fix to the tests in app. We unmarshal a RenderableNode struct,
which has a JSON-ignored NodeMetadata field. The zero value is invalid,
so we need to fix that before performing comparisons.

- tag/weave* -> package overlay
- tag/origin_host* -> package host
- tag/topology* -> package main

Also, add explicit GOOS=darwin target for circle.

- add tests
- use constants for NodeMetadata keys
- add host uptime and kernel version