Summary:
Since SeekForPrev (used by Prev) is not supported by HashSkipList when prefix is used, we disable it when stress testing HashSkipList.
- Change the default memtablerep to skip list.
- Avoid Prev() when memtablerep is HashSkipList and prefix is used.
Test Plan (on devserver):
```
$make db_stress
$./db_stress -ops_per_thread=10000 -reopen=1 -destroy_db_initially=true -column_families=1 -threads=1 -column_families=1 -memtablerep=prefix_hash
$# or simply
$./db_stress
$./db_stress -memtablerep=prefix_hash
```
Results must print "Verification successful".
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5942
Differential Revision: D18017062
Pulled By: riversand963
fbshipit-source-id: af867e59aa9e6f533143c984d7d529febf232fd7
c53db172a1