suggest race condition to users that trip F14's internal checks
suggest race condition to users that trip F14's internal checks

Summary:
Some of the CHECKs and DCHECKs in F14 are often tripped by users
that write to an F14 map or set from multiple threads without proper
concurrency control.  This diff adds a message to those locations that
suggests that they may have a race condition.

Reviewed By: yfeldblum

Differential Revision: D22826369

fbshipit-source-id: 8d5e96feea4dbb8dda404980d9a9d3412fb2eb64

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly