Add a setting to skip null counters

[johnynek]

We have observed NPEs for null counters at least two companies using scalding. We have not root-caused this issue or found a good fix, but previously set to ignore all null counters.

Since some people rely on counters this is not a great plan, so instead this patch makes ignoring an opt-in behavior.

closes #1732

related to #1716 and #1726

cc @tdyas @rstewart this will change the default to error on null again. When you upgrade you will need to opt in to silently dropping counters using scalding.counters.skipnull=true

[CLAassistant]

All committers have signed the CLA.

[ianoc]

lgtm