Use Node 8.4 for builds

restart eBPF tracking on error

fixes #2650

This includes:

- https://github.com/iovisor/gobpf/pull/70
  perf: close go channels idiomatically

- https://github.com/iovisor/gobpf/pull/70
  close channels on the sender side & fix closing race

- https://github.com/weaveworks/tcptracer-bpf/pull/50
  vendor: update gobpf

EbpfTracker can die when the tcp events are received out of order. This
can happen with a buggy kernel or apparently in other cases, see:
https://github.com/weaveworks/scope/issues/2650

As a workaround, restart EbpfTracker when an event is received out of
order. This does not seem to happen often, but as a precaution,
EbpfTracker will not restart if the last failure is less than 5 minutes
ago.

This is not easy to test but I added instrumentation to trigger a
restart:

- Start Scope with:
    $ sudo WEAVESCOPE_DOCKER_ARGS="-e SCOPE_DEBUG_BPF=1" ./scope launch

- Request a stop with:
    $ echo stop | sudo tee /proc/$(pidof scope-probe)/root/var/run/scope/debug-bpf

refactor: reduce duplication in links_test

remove extra indent on note

And get rid of B -> MB conversion

Fixes #2817

* scope/cortex: fix typo in query

* Get rid of the "controllers" code

They do not all share the same queries.

scope-app:
* Adds `-app.metrics-graph` cli flag for configuring the base url to
use for graph links; supports :orgID and :query placeholders
* Assigns query URLs to existing metrics and appends empty metrics if missing

scope-ui:
* Extends <CloudFeature /> with option alwaysShow
* Adds <CloudLink /> to simplify routing when in cloud vs not in cloud
* Links metric graphs in the ui's node details view for all k8s
toplogies and containers so far
* Tracks metric graph click in mixpanel `scope.node.metric.click`
* Uses percentages and MB for CPU/Memory urls
* Passes timetravel timestamp to cortex in deeplink

Make Scope's URL message more precise

Addresses part of #2304.

Time Travel: keep active node details panels up-to-date

make network calculation commutative

Time Travel: unmount in the shutdown() action

Make nodes graph animations a bit faster

Fixes timetravel timestamp input getting truncatated on OSX

Now supports 88:88:88T88:88:88Z!

Time Travel: remove the feature flag and make the availability depend on historic reports capability