This PR removes locking around commonly accessed node attributes that do
not need to be locked. The locking could cause nodes to TTL as the
heartbeat code path was acquiring a lock that could be held for an
excessively long time. An example of this is when Vault is inaccessible,
since the fingerprint is run with a lock held but the Vault
fingerprinter makes the API calls with a large timeout.

Fixes https://github.com/hashicorp/nomad/issues/2689

Fixes https://github.com/hashicorp/nomad/issues/3181

This PR fixes an issue introduced in Nomad 0.6.0 due to
https://github.com/shirou/gopsutil/issues/420. The issue arised from the
fact that the Windows stats from gopsutil reports CPUs in
percentages where we expected ticks.

This commit protects against finding `0.NaN` tokens in JSON streams
because of infinity representation on serialization.

move metrics to telemetry; copy to client config

better api example, add telemetry documentation

This PR fixes the server url passed to docker credential helpers and
fixes stderr capture.

Fixes https://github.com/hashicorp/nomad/issues/2957

* Parse Docker mounts correctly

This PR fixes the parsing of Docker mounts and adds testing to ensure no
regressions.

Fixes https://github.com/hashicorp/nomad/issues/3156

* Review feedback

To reproduce logspam:

```
$ docker plugin install --grant-all-permissions vieux/sshfs
$ nomad agent -dev
...
2017/08/25 17:09:03.282868 [WARN] client: error fetching host disk usage stats for /var/lib/docker/plugins/a8b4a69b07e5180f828d19e1e9e102ccc0e26f9c9939eaef85357260c30b20a7/rootfs/mnt/volumes: permission denied
... repeats every collection period ...
```

This PR changes the fingerprint handling of network interfaces that only
contain link local addresses. The new behavior is to prefer globally
routable addresses and if none are detected, to fall back to link local
addresses if the operator hasn't disallowed it. This gives us pre 0.6
behavior for interfaces with only link local addresses but 0.6+ behavior
for IPv6 interfaces that will always have a link-local address.

Fixes https://github.com/hashicorp/nomad/issues/3005

/cc diptanuc