Refactor folly::ConcurrentHashMapSegment to delegate internal work to BucketTable impl_ member

Summary:
This sets the stage for D14458269, which adds a 2nd possible ImplT class.

Move code shared between ImplT's into shared area.

Work done in *_internal functions and functions that access member fields other than `batch_` now goes into this impl_ member.

ConcurrentHashMapSegment basically is now just a wrapper that knows how to translate things like emplace, try_emplace, assign, etc. to various flavors of insert_internal. Changes to erase control flow are analogous.

Reviewed By: davidtgoldblatt

Differential Revision: D14458241

fbshipit-source-id: 41d103141ead1eb306b6fe863932f00f827f68c3