Objects cannot be updated if a bad CRC is encountered by Bitcask

Steps to reproduce:
1. Start Riak with Bitcask as the backend
2. Write an object
3. Introduce an error in the Bitcask data file
4. Update the value of the object

Expected result:
Object is updated to new value

Actual result:
Update fails on the vnode with the bad CRC error


The following branch on Github has an eunit test that demonstrates the issue:
https://github.com/basho/riak_kv/tree/bitcask-badcrc-eunit-test