Change from writing '\x00' to writing '\x7f', and verify that is the
byte that woke the sleeping thread. Add a bunch more IO logging.

Needed to make large range allocations (1000 per ALLOCATE_ID roundtrip)
feasible.

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.