Track step nesting in StreamEngine (Reland).

Track step nesting in StreamEngine (Reland).

Currently, step nesting is tracked by the step runner as an integer,
which is passed to the stream engine as the `set_nest_level` post-create
event. This poses two problems, namely:
- The nest level is known to the runner immediately on step creation,
  but is presented to the stream engine as a separate piece of
  information.
- This forces the step runner's concept of nesting (integer) onto the
  stream engine, when all it really cares about is whether or not a step
  is nested.

This patch updates the API to pass the nested state as a boolean: either
a step is nested in the previous step, or it isn't. The stream engine is
responsible for tracking how it represents this to the stream's
consumer. The nested state is also passed at step creation, making
nesting an atomic step propety instead of an out-of-band option.

Along the way, small cleanups.

BUG=chromium:628770
TEST=local
  - Ran unit tests locally.
  - `./recipes.py --package ./infra/config/recipes.cfg run example/nested`

Committed: https://github.com/luci/recipes-py/commit/227f56ba6353986a7fe6d56fb283ad73c84e94d9

[dnj, 2016-09-08 20:08:19]

dnj@chromium.org changed reviewers:
+ iannucci@chromium.org, martiniss@chromium.org

[dnj, 2016-09-08 20:08:19]

PTAL, reland of https://codereview.chromium.org/2245113002

This fixes the bug and adds a unit test that asserts the bug, as well as one
additional nesting criteria that seemed useful to assert.

[iannucci, 2016-09-08 22:24:17]

lgtm

[dnj, 2016-09-08 22:25:35]

The CQ bit was checked by dnj@chromium.org

[commit-bot: I haz the power, 2016-09-08 22:25:41]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-09-08 22:33:34]

Description was changed from

==========
Track step nesting in StreamEngine (Reland).

Currently, step nesting is tracked by the step runner as an integer,
which is passed to the stream engine as the `set_nest_level` post-create
event. This poses two problems, namely:
- The nest level is known to the runner immediately on step creation,
  but is presented to the stream engine as a separate piece of
  information.
- This forces the step runner's concept of nesting (integer) onto the
  stream engine, when all it really cares about is whether or not a step
  is nested.

This patch updates the API to pass the nested state as a boolean: either
a step is nested in the previous step, or it isn't. The stream engine is
responsible for tracking how it represents this to the stream's
consumer. The nested state is also passed at step creation, making
nesting an atomic step propety instead of an out-of-band option.

Along the way, small cleanups.

BUG=chromium:628770
TEST=local
  - Ran unit tests locally.
  - `./recipes.py --package ./infra/config/recipes.cfg run example/nested`
==========

to

==========
Track step nesting in StreamEngine (Reland).

Currently, step nesting is tracked by the step runner as an integer,
which is passed to the stream engine as the `set_nest_level` post-create
event. This poses two problems, namely:
- The nest level is known to the runner immediately on step creation,
  but is presented to the stream engine as a separate piece of
  information.
- This forces the step runner's concept of nesting (integer) onto the
  stream engine, when all it really cares about is whether or not a step
  is nested.

This patch updates the API to pass the nested state as a boolean: either
a step is nested in the previous step, or it isn't. The stream engine is
responsible for tracking how it represents this to the stream's
consumer. The nested state is also passed at step creation, making
nesting an atomic step propety instead of an out-of-band option.

Along the way, small cleanups.

BUG=chromium:628770
TEST=local
  - Ran unit tests locally.
  - `./recipes.py --package ./infra/config/recipes.cfg run example/nested`

Committed:
https://github.com/luci/recipes-py/commit/227f56ba6353986a7fe6d56fb283ad73c84...
==========

[commit-bot: I haz the power, 2016-09-08 22:33:38]

Committed patchset #3 (id:40001) as
https://github.com/luci/recipes-py/commit/227f56ba6353986a7fe6d56fb283ad73c84...