Summary:
When  expiration is set in a pessimistic transaction, `txn_state_` is already updated to `AWAITING_PREPARE` in the `if (expiration_time_ > 0)` block, there is  no need to update the state in `if (can_prepare)` block again.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6778

Test Plan: make check

Reviewed By: lth

Differential Revision: D21335319

Pulled By: cheng-chang

fbshipit-source-id: 251d634cc7d1a0e86e673a59f0bda8584da5a35f