Summary:
There's no need in the atomic refcount decrement from 1->0, because we're the last live reference and using it to bump 1->2 is already undefined behavior.

Added benchmark:

Before:
```
stepan@devvm329:~/fbsource/fbcode$ buck-out/opt/gen/folly/io/test/iobuf_benchmark --bm_min_usec=1000000
============================================================================
folly/io/test/IOBufBenchmark.cpp                relative  time/iter  iters/s
============================================================================
createAndDestroy                                            36.35ns   27.51M
cloneOneBenchmark                                           40.40ns   24.75M
cloneOneIntoBenchmark                                       27.03ns   37.00M
cloneBenchmark                                              42.23ns   23.68M
cloneIntoBenchmark                                          28.35ns   35.27M
moveBenchmark                                               15.20ns   65.79M
copyBenchmark                                               35.00ns   28.57M
cloneCoalescedBaseline                                     334.59ns    2.99M
cloneCoalescedBenchmark                          660.01%    50.69ns   19.73M
takeOwnershipBenchmark                                      50.57ns   19.78M
============================================================================
```

After:
```
stepan@devvm329:~/fbsource/fbcode$ buck-out/opt/gen/folly/io/test/iobuf_benchmark --bm_min_usec=1000000
============================================================================
folly/io/test/IOBufBenchmark.cpp                relative  time/iter  iters/s
============================================================================
createAndDestroy                                            30.04ns   33.29M
cloneOneBenchmark                                           41.27ns   24.23M
cloneOneIntoBenchmark                                       26.37ns   37.92M
cloneBenchmark                                              43.91ns   22.77M
cloneIntoBenchmark                                          28.49ns   35.10M
moveBenchmark                                               15.50ns   64.52M
copyBenchmark                                               35.85ns   27.89M
cloneCoalescedBaseline                                     318.49ns    3.14M
cloneCoalescedBenchmark                          643.69%    49.48ns   20.21M
takeOwnershipBenchmark                                      45.15ns   22.15M
============================================================================
```

Reviewed By: yfeldblum, davidtgoldblatt

Differential Revision: D14715579

fbshipit-source-id: 3c0373b8423dda680920860979cfa240bf3d8d7a