The 'may-i-run' endpoint may say 'no' to us running task X, but when the
response comes in and is handled the worker may already be working on task
Y. This case is now recognised, and the 'no' will be ignored.
This makes a clearer distinction between Flamenco tasks ('task') and
AsyncIO tasks ('future'). The AsyncIO Task class is a subclass of its
Future class.
This also means that we need a `tests/__init__.py` file; this file makes
py.test undertand what is our sources directory, and thus allows them to
`import flamenco_worker`. As a result, the test imports from tests/*.py
need to change to relative imports.
I'm guessing the do_db_push() async function execution was starved by
other asyncio tasks, causing Python to remember all the to-be-queued
payloads in memory. By making the function synchronous this doesn't happen.
When the "may I run" system says "no", we shouldn't push a "canceled"
status to the Manager, as this could overwrite another valid status that's
there for some other reason.
This is a clone of the Flamenco repository from back in the days when
Server, Manager and Worker shared the same Git repository. This is the
commit where that ended, and they went their separate ways.
