[nine] Asynchronous Steps

[Buildbot Friendly Helper]

This ticket is a migrated Trac ticket 2646

People contributed to the original ticket: @djmitche (commenter, reporter), @sa2ajj (commenter)

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In 0.8.x, build steps are very synchronous.
Nine introduces "new-style" steps that call methods asynchronously and do not expect easy access to the full text of logfiles.
However, there's also substantial work in place to maintain backward compatibility - at least temporarily - for old-style steps.
This project is mostly in Dustin's head.
Here are the remaining bits:

• implement new-style compatibility on master, as well

  • just return defer.succeed(..); no need for wrappers for old-style steps
  • include enough checks that users can be confident their new-style steps are OK

• change backend to use data API

  • persist properties at build or step finish
  • change addLog to return a [[ProcessLog]] instance

• ensure that new-style steps don't see any removed methods or old behaviors

  • all of the stuff listed as removed in the relnotes
  • createSummary, log_eval_func, among others

• rewrite steps to new style

  • but not ShellCommand, yet, since user classes derive from it
  • merge rewrites to master
  • remove step_status for statistics

• update documentation

  • lots of changes in customization.rst
  • cfg-statustargets.rst references getText

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