minor improvements to db_stress
minor improvements to db_stress

Summary:
fix some things that made this command hard to use from CLI:

- use default values for `target_file_size_base` and `max_bytes_for_level_base`. previously we were using small values for these but default value of `write_buffer_size`, which led to enormous number of L1 files.
- failure message for `value_size_mult` too big. previously there was just an assert, so in non-debug mode it'd overrun the value buffer and crash mysteriously.
- only print verification success if there's no failure. before it'd print both in the failure case.
- support `memtable_prefix_bloom_size_ratio`
- support `num_bottom_pri_threads` (universal compaction)
Closes https://github.com/facebook/rocksdb/pull/2741

Differential Revision: D5629495

Pulled By: ajkr

fbshipit-source-id: ddad97d6d4ba0884e7c0f933b0a359712514fc1d

SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb