Summary: Added RandomWithVerify benchmark option.

Test Plan:
This whole diff is to test.
[nponnekanti@dev902 /data/users/nponnekanti/rocksdb] ./db_bench --benchmarks=randomwithverify
LevelDB:    version 1.5
Date:       Tue Feb 19 17:50:28 2013
CPU:        24 * Intel(R) Xeon(R) CPU           X5650  @ 2.67GHz
CPUCache:   12288 KB
Keys:       16 bytes each
Values:     100 bytes each (50 bytes after compression)
Entries:    1000000
RawSize:    110.6 MB (estimated)
FileSize:   62.9 MB (estimated)
Compression: snappy
WARNING: Assertions are enabled; benchmarks unnecessarily slow
------------------------------------------------
Created bg thread 0x7fa9c3fff700
randomwithverify :       5.004 micros/op 199836 ops/sec; ( get:900000 put:80000 del:20000 total:1000000 found:711992)

Revert Plan: OK

Task ID: #

Reviewers: dhruba, emayanke

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D8685