Summary:
Currently the default txn write policy in crash tests is WRITE_PREPARED. The patch randomly picks the write policy at the start of the crash test.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6158
Test Plan:
```
make -j32 crash_test_with_txn
```
Differential Revision: D18946307
Pulled By: maysamyabandeh
fbshipit-source-id: f77d7a94f99a08791ef9626da153d284bf521950