As part of deprecating legacy drivers, we're moving the env package to a
new drivers/shared tree, as it is used by the modern docker and rkt
driver packages, and is useful for 3rd party plugins.

Saves a tiny bit of cpu and some IO. Sadly doesn't prevent all IO on
duplicate writes as the transactions are still created and committed.

$ go test -bench=. -benchmem
goos: linux
goarch: amd64
pkg: github.com/hashicorp/nomad/helper/boltdd
BenchmarkWriteDeduplication_On-4             500           4059591 ns/op           23736 B/op         56 allocs/op
BenchmarkWriteDeduplication_Off-4            300           4115319 ns/op           25942 B/op         55 allocs/op

Deregistration works but difficult to test due to terminal updates not
being fully implemented in the new client/ar/tr.

Guard against Canary being set to false at the same time as an
allocation is being stopped: this could cause RemoveTask to be called
with the wrong Canary value and leaking a service.

Deleting both Canary values is the safest route.

Also refactor Consul ServiceClient to take a struct instead of a massive
set of arguments. Meant updating a lot of code but it should be far
easier to extend in the future as you will only need to update a single
struct instead of every single call site.

Adds an e2e test for canary tags.

Needs to be more like http.

Adding this fields to the DriverContext object, will allow us to pass
them to the drivers.

An use case for this, will be to emit tagged metrics in the drivers,
which contain all relevant information:
- Job
- TaskGroup
- Task
- ...

Ref: https://github.com/hashicorp/nomad/pull/4185

For people not using driver networks these log lines would just be
confusing.

Previously if only an interpolated variable used in a service or check
was changed we interpolated the old and new services and checks with the
new variable, so nothing appeared to have changed.

Fixes the panic mentioned in
https://github.com/hashicorp/nomad/issues/3420#issuecomment-341666932

While a leader task dying serially stops all follower tasks, the
synchronizing of state is asynchrnous. Nomad can shutdown before all
follower tasks have updated their state to dead thus saving the state
necessary to hit this panic: *have a non-terminal alloc with a dead
leader.*

The actual fix is a simple nil check to not assume non-terminal allocs
leader's have a TaskRunner.

Remove event GenericSource, and address other code review comments. Also added deprecation info in comments.

Move logic for determinic event display message to task_runner, added two new fields DisplayMessage and Details.

@dadgar made the excellent observation in #3105 that TaskRunner removes
and re-registers checks on restarts. This means checkWatcher doesn't
need to do *any* internal restart tracking. Individual checks can just
remove themselves and be re-added when the task restarts.

Also expose a mock Consul Agent to allow testing ServiceClient and
checkWatcher from TaskRunner without actually talking to a real Consul.