Use batching to encrypt/decrypt fields

As noted in the code comments it would be good to use Batch API to encrypt/decrypt fields - particular with largish data sets (and to give the user progression feedback). Attached is a patch which changes the process as follows:-
1. Create a duplicate table (temp_fieldtablename) with the same schema definition as the table
2. Copy the current data from the field table into this temporary table
3. Encrypt/decrypt data from the temporary table and copy in batches back to the original table
4. Drop the temporary table.

This is not perfect if there is an error in the process of copying and no database transactions or anything good like that, but we did find it worked well for us on a Postgres install. Sorry no tests on MySQL as of yet.

The current module performs the process in one large database query which tended to out of memory on us.