progress: handle race conditions; fixes #81

[jedib0t]

@Haraguroicha found a bunch of race conditions and unsafe access of a lot of properties in Progress Writer and opened a PR with fixes for the same @ #81

The other PR is out of date with master branch and has not been re-based yet. So, this PR attempts to do the same things as #81 and some more:

• cleaner mutex usage and new Length* interfaces
• introduce a Make step to check for race conditions using the pre-built progress demo
• modify the Travis config to check and fail for race conditions with every run

Fixes #81.

[coveralls]

Coverage remained the same at 100.0% when pulling d281e8b on progress-race into 918cee6 on master.