Fix realtime measurement for multi-threaded benchmarks

[krzikalla]

I tried to quick-fix the wall-clock time (real time) as discussed in PR #946. This is a prerequiste for adding some tests in PR #1027.
However, the changes now seriously overlap with PR #946 (esp. the change in line

benchmark/src/benchmark_runner.cc

Line 125 in 5c25ad3

results.iterations += st.iterations();

).

I realized, that a lot of design questions must be answered before we can go on with #946 and this one.

1. Do we allow a different iteration count across threads. (IMHO no and I'd check this) If yes: what results are reported then? What are their meanings?
2. How is manual timing handled in multi-threaded benchmarks? IMHO the current implementation does not match the comment at

   benchmark/include/benchmark/benchmark.h

   Lines 553 to 554 in 5c25ad3

   	// For threaded benchmarks the final value will be set to the largest
   	// reported values.

   IMHO and according to the comment it should be handled similar to real time.
3. Which values are written in which mode and what is their meaning? (default/real time/manual time * default/ProcessCPUTime)

@koct9i : Maybe you can use my tests as a template for your PR.

[LebedevRI]

@krzikalla thank you for looking into this.