Statsd implementation of puller API

Internal implementation of the puller API. Registers pullers by putting
them in the kAllPullAtomInfo map. Implements the actual pull,
with condition variables to timeout.

Lastly, keys the kAllPullAtom info by a PullerKey, which is a uid and
atom id. However, the uid is just set to a default of -1 for now. I will
work the security implementation in a follow up CL.
Test: builds, boots
Test: I will write unit tests in the future. It's very difficult to
write any without StatsEvent being completed.

Change-Id: Id602dd297b6ba7df811e2d5ab2e77efc0684e418