Summary:
This is initial version. A few ways in which this could
be extended in the future are:
(a) Killing from more places in source code
(b) Hashing stack and using that hash in determining whether to crash.
    This is to avoid crashing more often at source lines that are executed
    more often.
(c) Raising exceptions or returning errors instead of killing

Test Plan:
This whole thing is for testing.

Here is part of output:

python2.7 tools/db_crashtest2.py -d 600
Running db_stress

db_stress retncode -15 output LevelDB version     : 1.5
Number of threads   : 32
Ops per thread      : 10000000
Read percentage     : 50
Write-buffer-size   : 4194304
Delete percentage   : 30
Max key             : 1000
Ratio #ops/#keys    : 320000
Num times DB reopens: 0
Batches/snapshots   : 1
Purge redundant %   : 50
Num keys per lock   : 4
Compression         : snappy
------------------------------------------------
No lock creation because test_batches_snapshots set
2013/04/26-17:55:17  Starting database operations
Created bg thread 0x7fc1f07ff700
... finished 60000 ops
Running db_stress

db_stress retncode -15 output LevelDB version     : 1.5
Number of threads   : 32
Ops per thread      : 10000000
Read percentage     : 50
Write-buffer-size   : 4194304
Delete percentage   : 30
Max key             : 1000
Ratio #ops/#keys    : 320000
Num times DB reopens: 0
Batches/snapshots   : 1
Purge redundant %   : 50
Num keys per lock   : 4
Compression         : snappy
------------------------------------------------
Created bg thread 0x7ff0137ff700
No lock creation because test_batches_snapshots set
2013/04/26-17:56:15  Starting database operations
... finished 90000 ops

Revert Plan: OK

Task ID: #2252691

Reviewers: dhruba, emayanke

Reviewed By: emayanke

CC: leveldb, haobo

Differential Revision: https://reviews.facebook.net/D10581