Write Hooks and Evictions

[nighelles]

Evictions are described in the two comments mentioned here: https://github.com/rethinkdb/rethinkdb/issues/5823#issuecomment-228455004

Write Hooks are described in the comments here: https://github.com/rethinkdb/rethinkdb/issues/5813

I'll add more detail to this issue when both are finished with review.