Summary:
This is a bunch of small improvements to `VersionEdit`. Namely, the patch

* Makes the names and order of variables, methods, and code chunks related
  to the various information elements more consistent, and adds missing
  getters for the sake of completeness.
* Initializes previously uninitialized stack variables.
* Marks all getters const to improve const correctness.
* Adds in-class initializers and removes the default ctor that would
  create an object with uninitialized built-in fields and call `Clear`
  afterwards.
* Adds a new type alias for new files and changes the existing `typedef`
  for deleted files into a type alias as well.
* Makes the helper method `DecodeNewFile4From` private.
* Switches from long-winded iterator syntax to range based loops in a
  couple of places.
* Fixes a couple of assignments where an integer 0 was assigned to
  boolean members.
* Fixes a getter which used to return a `const std::string` instead of
the intended `const std::string&`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6383

Test Plan: make check

Differential Revision: D19780537

Pulled By: ltamasi

fbshipit-source-id: b0b4f09fee0ec0e7c7b7a6d76bfe5346e91824d0