add an interface for bulk sending stats

[eklitzke]

This change is motivated by a similar interface that @sophiaxc and I made for an internal project. The idea for that project was that each iteration of this loop was sending a lot of statsd metrics namespaced by the project name, and so an interface where the connection had the prefix and could do bulk sending of stats was a big improvement.

[charlax]

Looks good! What do you think of also having a context that batches those stats?

[eklitzke]

This is a really good idea @charlax, I will send an updated PR.

[JeremyGrosser]

Please add a unit test or two, just so we've got some level of coverage.