RFC: Pluggable WriteController

I presented an alternate write throttle for slower class servers at the annual RocksDB Meetup December 4, 2017. The current implementation is completely external to RocksDB. During the Q&A session, I was asked how this could be better/cleanly integrated with RocksDB. My answer was to have the WriteController object pluggable, like the bloom filters and rate limiters. It was suggested that I post an RFC here first, then potentially submit a PR with said change and incorporating comments posted against the issue.

My suggested change is:

1. add a shared_ptr object to the Options structure where a user can supply a custom WriteController. This is a shared_ptr, not a unique_ptr, in case the user has multiple database instances that wish to share the same WriteController so as to have a "system-wide" view.

2. the shared_ptr from the Options structure is assigned to the DB_Impl object in place of the current WriteController member object. If the user does not supply a custom WriteController, an instance of the existing WriteController is automatically added.

3. write_controller.h moves to the include/rocksdb directory.

4. add "virtual" to most of the existing WriteController methods. This simplifies creating object derivatives.

5. optional: I have considered making some of the existing WriteController routines thread safe. They currently assume the caller holds the database mutex. This could be awkward if a derived instance is actually shared across multiple database instances. Otherwise, any user derived object will have to include the necessary protections.