[rocksdb] make init prefix more robust

Summary:
Currently if client uses kNULLString as the prefix, it will confuse
compaction filter v2. This diff added a bool to indicate if the prefix
has been intialized. I also added a unit test to cover this case and
make sure the new code path is hit.

Test Plan: db_test

Reviewers: igor, haobo

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17151