Summary: malloc_usable_size() gets a better estimation of memory usage. It is already used to calculate block cache memory usage. Use it in arena too.

Test Plan: Run all unit tests

Reviewers: anthony, kradhakrishnan, rven, IslamAbdelRahman, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D43317