This version is based on the modulefinder standard library module,
pruned back just to handle modules we know have been loaded already, and
to scan module-level imports only, rather than imports occurring in
class and function scope (crappy heuristic, but assume they are lazy
imports).

The ast and compiler modules were far too slow, whereas this version can
bytecode compile and scan all the imports for django.db.models (58
modules) in around 200ms.. 3.4ms per dependency, it's probably not going
to get much faster than that.

I'm fairly sure these updates are correct, but I'm not able to run the
test suite to confirm.

A previous commit renamed run_with_broker() and with_broker() to
run_with_router() and with_router() respctively. Some references were
missed.

Fixes an undefined variable error found by pylint

Fixes an undefined variable error found by pylint

Found with pylint

Found using pylint

Found using pylint

Found using pylint

This probably worsens performance in the common case, but it prevents
runaway producers (see e.g. issue #36) from spending all their CPU
copying around huge strings.

It's also a small step towards a solution to issue #6, which will
replace the output buffer with some sort of fancier queue anyway.

This reduces a particular 40 second run of rsync to 1.5 seconds.

needed for inter-child rsync.

Saves an import, pain in the ass to type all the time.

Makes it easier to match up reads/writes across log files by using
message byte count

The last time I tested set_nonblock() as a fix for the rsync hang, I
used F_SETFD rather than F_SETFL, which resulted in no error, but also
did not set O_NONBLOCK. Turns out missing O_NONBLOCK was the problem.

The rsync hang was due to every context blocking in os.write() waiting
for either a parent or child buffer to empty, which was exacerbated by
rsync's own pipelining, that allows writes from both sides to proceed
even while reads aren't progressing. The hang was due to os.write() on a
blocking fd blocking until buffer space is available to complete the
write. Partial writes are only supported when O_NONBLOCK is enabled.