expose FunctionScheduler::addFunction() with a custom distribution

expose FunctionScheduler::addFunction() with a custom distribution

Summary:
Make addFunctionInternal() publicly available, as an overloaded version of
addFunction().  This allows users to add functions with a specified poisson
distribution.

This allows us to deprecate our internal legacy version of FunctionScheduler,
and replace it with the folly version.

Test Plan: Confirmed all unit tests still pass.

Reviewed By: ldbrandy@fb.com

Subscribers: jwatzman, doug, net-systems@, exa, folly-diffs@, yfeldblum, chalfant

FB internal diff: D2051699

Signature: t1:2051699:1431379841:f3547d1ed371503b0bf91b509a4ef03e881aa991