Use cbegin and cend in static reflection container traits
Use cbegin and cend in static reflection container traits

Summary:
[Thrift] Use `cbegin` and `cend` in static reflection container traits.

These are functions that all the C++ standard library containers have.

And add the two methods to `folly::sorted_vector_set` and `folly::sorted_vector_map`.

Reviewed By: juchem

Differential Revision: D3763187

fbshipit-source-id: 9d467a1cf391206eba671f454da428323c4a566a

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