Running bazel test //test/exe:main_common_test --runs_per_test=1000 fails 24/1000 times.

[2019-01-28 03:38:22.276][15][critical][assert] [source/common/thread/thread_factory_singleton.cc:16] assert failure: (thread_factory == nullptr) != (thread_factory_ == nullptr).

IMO it would be cleaner to just plumb Api::Api& where it needs to go so we don't need a prod singleton. We will likely need it almost everywhere anyway; it will be carrying TimeSystem/TimeSource as well once #5660 is merged, and will be available anywhere Filesystem methods are used once #5692 goes in.