[Reviewer: Ellie] Fix crest issue #162

In the diffs it looks like src/metaswitch/crest/api/base.py had the existing Accumulator, Counter imports. I don't want you to remove these (although I do want you to change how you're importing monotonic_time so it doesn't come from statistics). 

In src/metaswitch/crest/test/api/base.py you've added the Accumulator, Counter imports. These shouldn't be needed.