More bug squashing

[dandanlen]

4 commits here:

• [FIX] Don't attempt to generate a value distribution if the column contains only null values.
• [FIX] Evaluate list of metrics strictly instead of lazily. I think a side-effect of this was that exceptions were being thrown in the wrong place and not being caught / forwarded to sentry / included in the errors returned via the api.
• [FIX] Group string lengths correctly to prevent duplicates.
• [DEV] Add sentry server name in devcontainer.

Fixes #282

[sebastian]

Seems Travis isn't happy. Reran the failing build once to see if that could help, but it didn't rectify it. https://travis-ci.com/github/diffix/explorer/builds/179288451

[dandanlen]

I am not quite sure I follow the change regarding making the system non-lazy in evaluating the stats. Superficially the laziness sounds like it would be an advantage by potentially not executing analyses unless they are needed?

We need to evaluate the results strictly/eagerly from the api consumer's perspective ie. we need to drive the results to completion in the background task, not when the /result api endpoint is hit. In this case the laziness in question only impacted the 'shaping' of the data (and only for some metrics) but I wanted to make everything consistent and evaluate these objects to minimise the response time via /result.

I reran travis this morning and it worked - there were some bad gateway responses yesterday that caused the failing tests.