move metrics to telemetry; copy to client config

better api example, add telemetry documentation

interface has 3 implementations:

1. local for blocking and moving data locally
2. remote for blocking and moving data from another node
3. noop for allocs that don't need to block

Plus some logging improvements that may help with #2563

Fixes #2685

Also inject PATH into rkt commands since we're no longer appending host
env vars for it.

More than gc_max_allocs may be running on a node, but terminal allocs
will be garbage collected to try to keep the total number below the
limit.

This reverts commit 4d6a012c.

Fixes #2478 #2474 #1995 #2294

The new client only handles agent and task service advertisement. Server
discovery is mostly unchanged.

The Nomad client agent now handles all Consul operations instead of the
executor handling task related operations. When upgrading from an
earlier version of Nomad existing executors will be told to deregister
from Consul so that the Nomad agent can re-register the task's services
and checks.

Drivers - other than qemu - now support an Exec method for executing
abritrary commands in a task's environment. This is used to implement
script checks.

Interfaces are used extensively to avoid interacting with Consul in
tests that don't assert any Consul related behavior.

This PR takes the host ID and runs it through a hash so that it is well
distributed. This makes it so that machines that report similar host IDs
are easily distinguished.

Instances of similar IDs occur on EC2 where the ID is prefixed and on
motherboards created in the same batch.

Fixes https://github.com/hashicorp/nomad/issues/2534

This PR adds tracking to when a task starts and finishes and the logs
API takes advantage of this and returns better errors when asking for
logs that do not exist.

This PR fixes an issue in which the reference count on a Docker image
would become inflated through task restarts.