Currently when a string is passed to the parallel engine
it will only know how to create a process or a native thread
based executor. The futurist library also supports making
a green thread based executor, so support creating it.

This will save glance some code that they have to create
a executor based on different options (one of those is a
green option).

Change-Id: I15c164a38b4445d28eb6062aed6c56cce0e0364b

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

It appears folks are using the taskflow.utils code in there own
code-bases (likely taking it from the examples) which we do not
want to encourage, so remove the usage of **most** of
taskflow.utils code from the examples so that people are less
likely to copy/paste/reference it.

Change-Id: I0ce3c520de347e3e746e7912aa1366a515458424

Change-Id: If89baa042695f19e42b6368034f3ccf22c2cf0aa

Some things that popped out while reading the
comments/documentation.

Change-Id: I0ccecae3381447ede44bb855d91f997349be1562

Recreate a very simple eventlet utility module that
has only a few features; one function checks if eventlet is
available and if not raise an exception; and a constant that
can be used by calling code (such as tests or other optional
functionality) to check if eventlet is useable before
proceeding.

Change-Id: I32df0702eeae7c7c78972c9796156dd824b2f123

Change-Id: I1d6e6535ab09d7f6c9d9ca3e2663983644b7a8a1