Summary:
Add asserts to show the intentions of result explicitly.
Add examples to show the effect of optimistic transaction more clearly.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6074
Test Plan: `cd examples && make optimistic_transaction_example && ./optimistic_transaction_example`
Differential Revision: D18964309
Pulled By: cheng-chang
fbshipit-source-id: a524616ed9981edf2fd37ae61c5ed18c5cf25f55