Summary:
Remove db_impl_debug from NDEBUG, but allow it in ROCKSDB_LITE
These functions by definition should not be included in NDEBUG and they are only used for testing
This is based on offline discussion with @yhchiang and @igor

Test Plan:
make static_lib
make check

Reviewers: igor, sdong, yhchiang

Reviewed By: yhchiang

Subscribers: igor, yhchiang, dhruba

Differential Revision: https://reviews.facebook.net/D48573