[Java] Enable compression_ratio option in DbBenchmark.java

Summary:
Enable the random values in Java DB Bench to be generated based
on the compression_ratio specified in the command-line arguments.

Test Plan:
make rocksdbjava
java/jdb_bench.sh

Reviewers: sdong, ankgup87, haobo

Reviewed By: haobo

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19101
Improve an assertion in RandomGenerator::Generate() in db_bench.

Summary:
RandomGenerator::Generate() currently has an assertion len < data_.size().
However, it is actually fine to have len == data_.size().
This diff change the assertion to len <= data_.size().

Test Plan:
make db_bench
./db_bench

Reviewers: haobo, sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19269
Allow compaction to reclaim storage more effectively.

Summary:
This diff allows compaction to reclaim storage more effectively.
In the current design, compactions are mainly triggered based on
the file sizes.  However, since deletion entries does not have
value, files which have many deletion entries are less likely
to be compacted.  As a result, it may took a while to make
deletion entries to be compacted.

This diff address issue by compensating the size of deletion
entries during compaction process: the size of each deletion
entry in the compaction process is augmented by 2x average
value size.  The diff applies to both leveled and universal
compacitons.

Test Plan:
develop CompactionDeletionTrigger
make db_test
./db_test

Reviewers: haobo, igor, ljin, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19029
Reorder the member variables of FileMetaData to improve cache locality.

Summary:
Move stats related member variables of FileMetaData to the bottom to
improve cache locality of normal DB operations.

Test Plan: make

Reviewers: haobo, ljin, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19287