Turns out it is far too easy to burn through available file descriptors,
so try something else: self-pipes are per thread, and only temporarily
associated with a Lack that wishes to sleep.

Reduce pointless locking by giving Latch its own queue, and removing
Queue.Queue() use in some places.

Temporarily undo merging of of Waker and Latch, let's do this one step
at a time.

On Python 2.x, operations on pthread objects with a timeout set actually
cause internal polling. When polling fails to yield a positive result,
it quickly backs off to a 50ms loop, which results in a huge amount of
latency throughout.

Instead, give up using Queue.Queue.get(timeout=...) and replace it with
the UNIX self-pipe trick. Knocks another 45% off my.yml in the Ansible
examples directory against a local VM.

This has the potential to burn a *lot* of file descriptors, but hell,
it's not the 1940s any more, RAM is all but infinite. I can live with
that.

This gets things down to around 75ms per playbook step, still hunting
for additional sources of latency.

Now featuring one roundtrip per module invocation.

Fix a MyPy warning by only passing lists to select.select(). At least on
Python 2.x, select.select() was internally converting the sets to lists
anyway.

By the time lists become inefficient here, it is likely that
select.select() itself will also be inefficient, and need replaced with
.poll() or similar.

No discernible performance different when transferring django.db.models
to a local VM.

* Children should never generate a request for a module that has already
  been sent, however there are a variety of edge cases where, e.g.
  asynchronous calls are made into unloaded modules in a set of
  children, causing those children to request modules (and deps) in a
  different order, which might break deduplication. So add a warning to
  catch when this happens, so we can figure out how to handle it.
  Meanwhile it's only a warning since in the worst case, this just adds
  needless latency.

* Don't bother treating sent packages separately, there doesn't seem to
  be any need for this (after docs are updated to match how preloading
  actually works now).

Overwriting 'fullname' variable caused basically nonsensical filtering.
Result was including the module being searched in the list of
dependencies, which was causing ModuleResponder to send it early, which
was causing contexts to start importing the module before preloading of
dependencies had completed.

* SIGTERM safety net prevents profiler from writing results, so disable
  it when profiling is active.
* fix warning corrupting stream when profiling=True

Knocks 4kb off network footprint for a proxy connection.

Previously we'd send just None in GET_MODULE reply, but now since there
is no single request-reply structure, we must include the fullname in
the LOAD_MODULE response and make all of its data fields None to
indicate the same.

Doesn't yet implement the rules in the docs, but I think the doc rules
could maybe change to match this. Needs lots of cleanup work and
thorough testing, but this is a great start.