Add StatsD/Datadog integration

This should be fairly straightforward, I'll add comments inline wherever needed.
This PR needs https://github.com/boltdb/bolt/pull/212

@benbjohnson 

/cc @charlescng @snormore 
Emitting both the counts and the histogram is redundant, but I'm not sure what will be more useful in DD, so I figured let's try both for now and see. I'm not entirely sure I'm doing the histogram correctly.