FunctionScheduler - set running_ when it has actually started running

FunctionScheduler - set running_ when it has actually started running

Summary:
When start() throws because of reasons (in my case it was std::system_error("Resource temporarily unavailalble") coming from std::thread constructo)
and during the exception propagation we will have to destroy FunctionScheduler then
thread_.join() in shutdown will throw again.
This diff sets running_ after the thread is created.

Reviewed By: yfeldblum

Differential Revision: D4469816

fbshipit-source-id: cde54dfbf39f04d3ea9dfa02a65295f5440e5ea4