Summary:
* Dump blob db options to info log
* Remove BlobDBOptionsImpl to disallow dynamic cast *BlobDBOptions into *BlobDBOptionsImpl. Move options there to be constants or into BlobDBOptions. The dynamic cast is broken after #2645
* Change some of the default options
* Remove blob_db_options.min_blob_size, which is unimplemented. Will implement it soon.
Closes https://github.com/facebook/rocksdb/pull/2671

Differential Revision: D5529912

Pulled By: yiwu-arbug

fbshipit-source-id: dcd58ca981db5bcc7f123b65a0d6f6ae0dc703c7