Improve ldb consistency checks (#6802)

Improve ldb consistency checks (#6802)

Summary:
When using ldb, users cannot turn on force consistency check in most commands, while they cannot use checksonsistnecy with --try_load_options. The change fixes both by:
1. checkconsistency now calls OpenDB() so that it gets all the options loading and sanitized options logic
2. use options.check_consistency_checks = true by default, and add a --disable_consistency_checks to turn it off.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6802

Test Plan: Add a new unit test. Some manual tests with corrupted DBs.

Reviewed By: pdillinger

Differential Revision: D21388051

fbshipit-source-id: 8d122732d391b426e3982a1c3232a8e3763ffad0