fix Futex when steady_clock and system_clock precisions differ

fix Futex when steady_clock and system_clock precisions differ

Summary:
To handle the strange relationship between steady_clock and
system_clock (on some platforms these represent only one type, on some
platforms they are separate) Futex::futexWaitUntil converts the deadline
to a duration and back.  On Xcode 6 system_clock is measured in
microseconds and steady_clock in nanoseconds, resulting in a compilation
failure.  This diff fixes it.

Test Plan:
1. compile snippet manually using Xcode
2. new unit test that causes the same implicit conversion failure in a slightly different way

Reviewed By: mssarang@fb.com

Subscribers: trunkagent, folly-diffs@

FB internal diff: D1740903

Tasks: 5831196

Signature: t1:1740903:1418754770:32c999abf1dc87415ffebf45083a903abbded9f2