Fixes support for virtualenv. Closes #152.

It knows how to dispatch messages from multiple receivers (associated
with multiple services) to multiple threads, where the service
implementation is invoked on the message.

It wakes a maximum of one thread per received message.

It knows how to shut down gracefully.

Implication: due to the latch use, there are 2 file descriptors burned
for every thread. We don't need interruptibility here, so in future, it
might be nice to allow swapping a diferent queueing primitive into
Select (maybe a subclass?) just for this case.

Causes all threads sleeping on the select to wake.

This is to allow Select() to be used as a generic queueing primitive
that supports graceful shutdown.

Will be used more heavily for CI later, but it's already in use by
gcloud-ansible-playbook.py.

This is a work in progress.

Usage:
  - insert a call to mitogen.debug() in the desired process
  - kill -USR2 that process
  - observe its controlling TTY produces thread stack dumps

Need to re-test with the lock held, else >1 threads can end up waiting
for lock then reopening the log repeatedly.

This allows us to write 128kb at a time towards SSH, but it doesn't help
with sudo, where the ancient tty layer is always used.

Implication: the entire message remains buffered until its last byte is
transmitted. Not wasting time on it, as there are pieces of work like
issue #6 that might invalidate these problems on the transmit path
entirely.

Rather than slowly build up a Python string over time, we just store a
deque of chunks (which, in a later commit, will now be around 128KB
each), and track the total buffer size in a separate integer.

The tricky loop is there to ensure the header does not need to be sliced
off the full message (which may be huge, causing yet another spike and
copy), but rather only off the much smaller first 128kb-sized chunk
received.

There is one more problem with this code: the ''.join() causes RAM usage
to temporarily double, but that was true of the old solution too. Shall
wait for bug reports before fixing this, as it gets very ugly very fast.

There is no penalty for just passing as much data to the OS as possible,
it is not copied, and for a non-blocking socket, the OS will just keep
buffer as much as it can and tell us how much that was.

Also avoids a rather pointless string slice.

Reduces the number of IO loop iterations required to receive large
messages at a small cost to RAM usage.

Note that when calling read() with a large buffer value like this,
Python must zero-allocate that much RAM. In other words, for even a
single byte received, 128kb of RAM might need to be written.
Consequently CHUNK_SIZE is quite a sensitive value and this might need
further tuning.

accept() (per interface) returns a non-blocking socket because the
listener socket is in non-blocking mode, therefore it is pure scheduling
luck that a connecting-in child has a chance to write anything for the
top-level processs to read during the subsequent .recv().

A higher forks setting in ansible.cfg was enough to cause our luck to
run out, causing the .recv() to crashi with EGAIN, and the multiplexer
to respond to the handler's crash by calling its disconnect method. This
is why some reports mentioned ECONNREFUSED -- the listener really was
gone, because its Stream class had crashed.

Meanwhile since the window where we're waiting for the remote process to
identify itself is tiny, simply flip off O_NONBLOCK for the duration of
the connection handshake. Stream.accept() (via Side.__init__) will
reenable O_NONBLOCK for the descriptors it duplicates, so we don't even
need to bother turning this back off.

A better solution entails splitting Stream up into a state machine and
doing the handshake with non-blocking IO, but that isn't going to be
available until asynchronous connect is implemented. Meanwhile in
reality this solution is probably 100% fine.

There is some insane unidentifiable Mitogen context (the local context?)
that instantly crashes with a higher forks setting. It appears to be
harmless, but meanwhile this naturally shouldn't be happening.