Summary: Using an iterator instead of the Get method, each thread goes through a portion of the database and verifies values by comparing to the shared state.

Test Plan:
./db_stress --db=/tmp/tmppp --max_key=10000 --ops_per_thread=10000

To test some basic cases, the following lines can be added (each set in turn) to the verifyDb method with the following expected results:

    // Should abort with "Unexpected value found"
    shared.Delete(start);

    // Should abort with "Value not found"
    WriteOptions write_opts;
    db_->Delete(write_opts, Key(start));

    // Should succeed
    WriteOptions write_opts;
    shared.Delete(start);
     db_->Delete(write_opts, Key(start));

    // Should abort with "Value not found"
    WriteOptions write_opts;
    db_->Delete(write_opts, Key(start + (end-start)/2));

    // Should abort with "Value not found"
    db_->Delete(write_opts, Key(end-1));

    // Should abort with "Unexpected value"
    shared.Delete(end-1);

    // Should abort with "Unexpected value"
    shared.Delete(start + (end-start)/2);

    // Should abort with "Value not found"
    db_->Delete(write_opts, Key(start));
    shared.Delete(start);
    db_->Delete(write_opts, Key(end-1));
    db_->Delete(write_opts, Key(end-2));

To test the out of range abort, change the key in the for loop to Key(i+1), so that the key defined by the index i is now outside of the supposed range of the database.

Reviewers: emayanke

Reviewed By: emayanke

CC: dhruba, xjin

Differential Revision: https://reviews.facebook.net/D13071