Co-Authored-By: Mahmood Ali <mahmood@hashicorp.com>

Add field "kind" to task for use in connect tasks

Revert CNI auto-download 

This reverts commit 0bd157cc.

This reverts commit 54ce4d1f.

This fixes the recent allocation rows to navigate when clicking within them
rather than just on the link, which matches the cursor behaviour.

nomad: add job status metrics

consul: add Connect structs

vault: fix deadlock in SetConfig

vault: fix race in accessor revocations

This is a defensive measure as SetConfig should only be called serially.

This seems to be the minimum viable patch for fixing a deadlock between
establishConnection and SetConfig.

SetConfig calls tomb.Kill+tomb.Wait while holding v.lock.
establishConnection needs to acquire v.lock to exit but SetConfig is
holding v.lock until tomb.Wait exits. tomb.Wait can't exit until
establishConnect does!

```
  SetConfig -> tomb.Wait
     ^              |
     |              v
  v.lock <- establishConnection
```

Refactor all Consul structs into {api,structs}/services.go because
api/tasks.go didn't make sense anymore and structs/structs.go is
gigantic.

Add more comments to clarify job.Stable field

When rendering a task consul template, ensure that only task environment
variables are used.

Currently, `consul-template` always falls back to host process
environment variables when key isn't a task env var[1].  Thus, we add
an empty entry for each host process env-var not found in task env-vars.

[1] https://github.com/hashicorp/consul-template/blob/bfa5d0e133688920afd1e012404f765182e3d5e0/template/funcs.go#L61-L75

Check if driver handle is nil before execing

Defend against tr.getDriverHandle being nil.  Exec handler checks if
task is running, but it may be stopped between check and driver handler
fetching.

Update go-getter@1.1.0 to fix #3491

driver/docker: use default network mode

driver/docker: support unix destination mount path in windows