Summary:
In current commit protocol of pessimistic transaction, if the transaction is not prepared before commit, the commit protocol implicitly assumes that the user wants to commit without prepare.

This PR adds TransactionOptions::skip_prepare, the default value is `true` because if set to `false`, all existing users who commit without prepare need to update their code to set skip_prepare to true. Although this does not force the user to explicitly express their intention of skip_prepare, it at least lets the user be aware of the assumption of being able to commit without prepare.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6775

Test Plan: added a new unit test TransactionTest::CommitWithoutPrepare

Reviewed By: lth

Differential Revision: D21313270

Pulled By: cheng-chang

fbshipit-source-id: 3d95b7c9b2d6cdddc09bdd66c561bc4fae8c3251