Summary:
Tweaks to `folly::DelayedInit`:
* No potential for ambiguity in placement-new by using `::new` and casting the address to `void*`.
* Helper functions `slot` and `store` simplify `try_emplace_with` and `try_emplace`.
* `try_emplace_with` and `try_emplace` are no longer `const`-qualified, removing the need for `mutable` qualifiers.

The emplacement members should not be `const`-qualified since they are not semantically non-modifying. The `const` qualifier signals semantic non-modification. While it is the case that there are customs around `const` qualifiers and concurrency, these customs are not the core meaning of `const`.

Reviewed By: praihan

Differential Revision: D25919779

fbshipit-source-id: 95e5c4b4abf5285fbb00dddda1192998baf6ee59