[Reviewer: Ellie] Fix crest issue #162

If I don't import Accumulator & Counter in base.py, I get this:
```
PYTHONPATH=src bash -c 'bin/python src/metaswitch/crest/main.py'
Traceback (most recent call last):
  File "bin/python", line 56, in <module>
    exec(compile(__file__f.read(), __file__, "exec"))
  File "src/metaswitch/crest/main.py", line 48, in <module>
    from metaswitch.crest import api
  File "/var/tmp/fork.sync/crest/src/metaswitch/crest/api/__init__.py", line 35, in <module>
    from metaswitch.crest.api import base
  File "/var/tmp/fork.sync/crest/src/metaswitch/crest/api/base.py", line 202, in <module>
    latency_accumulator = Accumulator("P_latency_us")
NameError: name 'Accumulator' is not defined
```