Don't update indexes if index values have not changed

[jonahcohen]

Don't update indexes if index values have not changed

[spetrunia]

There are two ways to detect that the columns haven't changed:

• Look at table->write_set and find which columns are written. If some index doesn't have any columns that are written, it doesn't need to be updated
  ** We don't need to make the check for every row, it is sufficient to do it per-query (todo is it the same as per-query or different?)
  ** Need to run a few examples to check that SQL layer indeed sets table->write_set to those columns that are updated
• Right before issuing rocksdb->Put()/rocksdb->Delete() calls, check if we're going to write the same value as the one that's already there.

[yoshinorim]

If it works as expected, I like the first approach.

[spetrunia]

Implemented the first approach and committed here: https://reviews.facebook.net/D33471