Resolve race on approx_time_ in FastClock.

InitType should hold a lock before storing to approx_time_, which is later
read by the background worker.  When the worker is actively running (i.e., not
blocked on bg_cond_) it holds bg_mutex_.

InitType is called during benchmark setup only, so any contention induced for
the mutex should not have performance/accuracy consequences.
Merge pull request #23 from ckennelly/clock_initialization

Resolve race on approx_time_ in FastClock.