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.

In this commit, we add two driver interfaces for supporting `nomad exec`
invocation:

* A high level `ExecTaskStreamingDriver`, that operates on io reader/writers.
  Drivers should prefer using this interface
* A low level `ExecTaskStreamingRawDriver` that operates on the raw stream of
  input structs; useful when a driver delegates handling to driver backend (e.g.
  across RPC/grpc).

The interfaces are optional for a driver, as `nomad exec` support is opt-in.
Existing drivers continue to compile without exec support, until their
maintainer add such support.

Furthermore, we create protobuf structures to represent exec stream entities:
`ExecTaskStreamingRequest` and `ExecTaskStreamingResponse`.  We aim to reuse the
protobuf generated code as much as possible, without translation to avoid
conversion overhead.

`ExecTaskStream` abstract fetching and sending stream entities.  It's influenced
by the grpc bi-directional stream interface, to avoid needing any adapter.  I
considered using channels, but the asynchronisity and concurrency makes buffer
reuse too complicated, which would put more pressure on GC and slows exec operation.

This adds `alloc-exec` capability to allow operator to execute command into a
running task.  Furthermore, it adds `alloc-node-exec` capability, required when
the alloc task is raw_exec or a driver with no FSIsolation.

Extract command parsing and execution mocking into a separate struct.  Also,
allow mocking of different fs_isolation for testing.

Test helper that allows registration of jobs when ACL is activated.

This helper returns the token as well as the ACL policy, to be used in a later
commit for logging the token info associated with nomad exec invocation.

Fixed typo

Update CHANGELOG and version.go to reflect 0.9.1/0.9.2 split

Fixed typo: "dots wil be"/"dots will be".

yishan/revised readme

Fixed Typo

Fixed typo: scheduilng/scheduling.

divest /api from rest of nomad internal packages

The API package needs to be independent from rest of nomad packages, to
avoid leaking internal packages and dependencies (e.g. raft, ugorji,
etc)

client fingerprinter doesn't overwrite manual configuration

drivers/docker: Support volumes field in Windows

Docs for `nomad alloc signal`

allocs: Add nomad alloc signal command

logmon: recover from shutting down call locally