Summary:
There are two types of licenses used in folly, only one of
which is accepted by the linter.

This diff changes the license notices in pre-existing folly files!
Many folly/io/async/* files have the second type of license, but
without a Copyright notice. I have added copyright notices to these
files.

I have also added a compliant notice to
folly/test/function_benchmark/benchmark_impl.h, which was the sole file
in folly/test/function_benchmark/ that didn't have a standard license.

Test Plan:
The changes to folly are comment only.

Run all of folly against the linter, see no more license errors.

Reviewed By: davejwatson@fb.com

Subscribers: trunkagent, sdwilsh, njormrod, folly-diffs@, sjenkins

FB internal diff: D1648489

Tasks: 5486739

Signature: t1:1648489:1415035522:3d8bd9611eb7c7117b70d5e7f68de5768639a727