Summary:
The intention of the example for read committed is clearer with these added asserts.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6055
Test Plan: `cd examples && make transaction_example && ./transaction_example`
Differential Revision: D18621830
Pulled By: riversand963
fbshipit-source-id: a94b08c5958b589049409ee4fc4d6799e5cbef79