ldb: support --fix_prefix_len

Summary:
ldb to support --fix_prefix_len to allow us to verify more cases.
Also fix a small issue that --bloom_bits might not be applied if --block_size is not given.

Test Plan: run ldb tool against an example DB.

Reviewers: ljin, yhchiang, rven, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D24819