This change switches the conductor to using Futurist library executor and
periodic tasks worker instead of oslo.service periodic tasks. This allows
running periodic tasks in parallel and relying on more standard interfaces
(futures, executors) when dealing with asynchronous execution.
A green thread executor is used instead of using an eventlet green pool
directly. The maximum number of workers is taken from the existing
workers_pool_size configuration option, and no tasks are allowed
to be enqueued to mimic the previous behaviour (might be lifted later).
The periodic tasks worker is using the same executor, and its main loop thread
is also running on it. For this reason minimum value for workers_pool_size
is now 3: periodic task main loop, keep alive thread and at least one thread for
other tasks. A special decorator for driver-periodic tasks is now deprecated,
as the generic decorator can be used there as well.
Closes-Bug: #1526277
Change-Id: I57bf7cebfb6db805b6c521bacfef2993b16ce1ee
3429e3824c