Summary:
This is a serious bug. If paranod_check == true and WAL is corrupted, we don't fail DB::Open(). I tried going into history and it seems we've been doing this for a long long time.

I found this when investigating t5852041.

Test Plan: Added unit test to verify correct behavior.

Reviewers: yhchiang, rven, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D30597