This library no longer supports Python 2, thus usage of six can be
removed. This also removes workaround about pickle library used in
Python 2 only.

Change-Id: I19d298cf0f402d65f0b142dea0bf35cf992332a9

The networkx 2.x series has been out for two years now and supports
python 3.6 and greater[1]. This patch updates TaskFlow to require
a minimum of networkx 2.1. It also updates the code to support
recent deprecation expiration introduced in the 2.4 release.

[1] https://networkx.github.io/documentation/stable/news.html

Change-Id: Ife31d353ba80824ebc63c8b21ee90943badc8da3

It can be quite nice to expose a basic set of metrics
about the internals of an engine, including the time
in each state, and how long the engine is active for
and likely more in the future.

To start add a engine statistics property and gather
some basic timing data and place this data into this
property for access (and/or introspection) by users.

Part of blueprint gather-engine-statistics

Change-Id: Ibc3c78755bd8ae779b52fc4772519f243a521576

Instead of the added complexity of discarding flow nodes
we can simplify the compilation process by just retaining
them and jumping over them in further iteration and graph
and tree runtime usage.

This change moves toward a model that does just this, which
makes it also easier to in the future use the newly added
flow graph nodes to do meaningful things (like use them as
a point to change which flow_detail is used).

Change-Id: Icb1695f4b995a0392f940837514774768f222db4

Just directly use the built machine in the action engine and
avoid having another layer of abstraction that does not provide
that much value. This makes the code cleaner, and more easy to
understand (and so-on).

Change-Id: Iae1279098112254338258c1941c15889f1ad1a79