Implements streamign exec handling in both executors (i.e. universal and
libcontainer).

For creation of TTY, some incidental complexity leaked in.  The universal
executor uses github.com/kr/pty for creation of TTYs.

On the other hand, libcontainer expects a console socket and for libcontainer to
create the underlying console object on process start.  The caller can then use
`libcontainer.utils.RecvFd()` to get tty master end.

I chose github.com/kr/pty for managing TTYs here.  I tried
`github.com/containerd/console` package (which is already imported), but the
package did not work as expected on macOS.

Prepare executor to handle streaming exec API calls that reuse drivers protobuf
structs.

nomad exec part 1: plumbing and docker driver

Adds nomad exec support in our API, by hitting the websocket endpoint.

We introduce API structs that correspond to the drivers streaming exec structs.

For creating the websocket connection, we reuse the transport setting from api
http client.

Add consolidated testing package to serve as conformance tests for all drivers.

This adds a websocket endpoint for handling `nomad exec`.

The endpoint is a websocket interface, as we require a bi-directional
streaming (to handle both input and output), which is not very appropriate for
plain HTTP 1.0. Using websocket makes implementing the web ui a bit simpler. I
considered using golang http hijack capability to treat http request as a plain
connection, but the web interface would be too complicated potentially.

Furthermore, the API endpoint operates against the raw core nomad exec streaming
datastructures, defined in protobuf, with json serializer.  Our APIs use json
interfaces in general, and protobuf generates json friendly golang structs.
Reusing the structs here simplify interface and reduce conversion overhead.

Add a client streaming RPC endpoint for processing nomad exec tasks, by invoking
the relevant task handler for execution.

Also add a helper that converts the adapts the high level interface to the
low-level interface of nomad exec interfaces.

Revised NomadProject Structure

remove Index field from EmitNodeEventsResponse

Removed Terraform mention in integrations without existing guide.

copied consts used by jobspec parsing from structs into api package, to avoid referencing structs package in jobspec (#5663)

Consul Catalog Integration Fixes

docs: Change querystring -> query string

- Revised "What is Nomad" copy
- Added "Key Features" section with links to task drivers & device plugins with lift-and-shift from README
- Added "Who Uses Nomad" section with users, talks, blog posts
- Removed Hadoop YARN, Docker Swarm, HTCondor from comparisons
- Revamped Guides section
- Inserted "Installing Nomad", "Upgrading", "Integrations" as persistent in Guides navbar
- Split Installing Nomad into two paths for users (one for Sandbox with "Quickstart", one for Production)
- Surfaced "Upgrading" and "Integrations" section from documentation
- Changed "Job Lifecycle" section into "Deploying & Managing Applications"
- Reworked "Operations" into "Operating Nomad"
- Reworked "Security" into "Securing Nomad"
- Segmented Namespaces, Resource Quotas, Sentinel into "Governance & Policy" subsection
- Reworked "Spark integration" into its own "Analytical Workloads" section

Remove unnecessary locking and serverlist syncing in heartbeats

* Typo fixes
* it's -> its
* plugable -> pluggable

`Index` is already included as part of `WriteMeta` embedding.

This is a backward compatible change: Clients never read the field; and
Server refernces to `EmitNodeEventsResponse.Index` would be using the
value in `WriteMeta`, which is consistent with other response structs.

* client: was not using up-to-date client state in determining which alloc count towards allocated resources

* Update client/client.go
Co-Authored-By: cgbaker <cgbaker@hashicorp.com>

docs: fix default restart values

Fixed Typo

Fixed typo: schedular/scheduler.