driver plugin loader

client: fingerprint driver plugins

Deprecate allocrunner and promote allocrunnerv2 to allocrunner

driver plugin: raw_exec implementation

If ar.state.TaskStates has not been set, set it on the copy of ar.state.
That keeps ar.state manipulations in one location and allows AllocState
to only acquire read-locks.

Although the really exciting change is making WaitForRunning return the
allocations that it started. This should cut down test boilerplate
significantly.

Sadly can't move the fingerprint timeout tweak into the helper due to
circular imports.

Also expose NoopDB for use in tests.

The interesting decision in this commit was to expose AR's state and not
a fully materialized Allocation struct. AR.clientAlloc builds an Alloc
that contains the task state, so I considered simply memoizing and
exposing that method.

However, that would lead to AR having two awkwardly similar methods:
 - Alloc() - which returns the server-sent alloc
 - ClientAlloc() - which returns the fully materialized client alloc

Since ClientAlloc() could be memoized it would be just as cheap to call
as Alloc(), so why not replace Alloc() entirely?

Replacing Alloc() entirely would require Update() to immediately
materialize the task states on server-sent Allocs as there may have been
local task state changes since the server received an Alloc update.

This quickly becomes difficult to reason about: should Update hooks use
the TaskStates? Are state changes caused by TR Update hooks immediately
reflected in the Alloc? Should AR persist its copy of the Alloc? If so,
are its TaskStates canonical or the TaskStates on TR?

So! Forget that. Let's separate the static Allocation from the dynamic
AR & TR state!

 - AR.Alloc() is for static Allocation access (often for the Job)
 - AR.AllocState() is for the dynamic AR & TR runtime state (deployment
   status, task states, etc).

If code needs to know the status of a task: AllocState()
If code needs to know the names of tasks: Alloc()

It should be very easy for a developer to reason about which method they
should call and what they can do with the return values.

httptest.ResponseRecorder exposes a bytes.Buffer which we were reading
and writing concurrently to test streaming log APIs. This is a race, so
I wrapped the struct in a lock with some helpers.