memrchr and *timed_mutex are platform-specific

Summary:
- conditionally compile rfind overrides
- conditionally add support for timed_mutex/recursive_timed_mutex

Test Plan:
- compiled on OSX
- unit tests

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D872272
move assignment operators for folly::Synchronized

Summary:
* added move assignment operators,
* fixed `operator=(const Synchronized& rhs)` (it had a typo),
* fixed deadlock on self-assignment,
* changed `swap` to call `swap(lhs.datum_, rhs.datum_)` instead of `lhs.datum_.swap(rhs.datum_)`.

Test Plan: fbconfig -r folly/test && fbmake opt -j32 && fbmake runtests_opt

Reviewed By: delong.j@fb.com

FB internal diff: D875977
guard<Exception>()

Summary: For handling exceptions from downstream operations.

Test Plan: Unit tests

Reviewed By: marcelo.juchem@fb.com

FB internal diff: D874344
Revert D874344 (it broke the build)

Summary:
Clean revert.  Please be more careful; phabricator warned you
these would break the build.

Test Plan: build something that wasn't working

Reviewed By: myang@fb.com

FB internal diff: D877526
Allowing trailing comma in folly::parseJson

Summary:
Introduced a new serialization option, `allow_trailing_comma`, which allows trailing commas to be included in strings when they are parsed. This isn't strictly allowed by RFC 4627, but we're allowing more than that anyway.

I've run into this dozens of times, especially while using SMC.

Test Plan: Unit tests

Reviewed By: delong.j@fb.com

FB internal diff: D872581