serial/parallel benchmark

serial/parallel benchmark

Summary: This is an attempt to see the effect of lock contention in the less-common case where a promise is fulfilled at the same time the future is still being set up. Although this is expected to be off the beaten path, it will still be a good idea to keep an eye on how we improve or worsen perf, when we play with different locking strategies (which I intend to do a lot of in the near term).

Test Plan:
============================================================================
folly/wangle/test/Benchmark.cpp                 relative  time/iter  iters/s
============================================================================
constantFuture                                             249.31ns    4.01M
promiseAndFuture                                 100.88%   247.13ns    4.05M
withThen                                          43.87%   568.30ns    1.76M
----------------------------------------------------------------------------
oneThen                                                    569.62ns    1.76M
twoThens                                          63.46%   897.60ns    1.11M
fourThens                                         39.64%     1.44us  695.90K
hundredThens                                       1.91%    29.75us   33.61K
----------------------------------------------------------------------------
serial                                                       4.97ms   201.21
parallel                                          60.22%     8.25ms   121.18
============================================================================

Reviewed By: jsedgwick@fb.com

Subscribers: meisner, net-systems@, fugalh, exa, njormrod

FB internal diff: D1593010