Define INSERT/UPDATE constants for hook_field_storage_write

Thanks for the patch! Review:

* The INSERT and UPDATE constants are really just unique identifiers. In other languages they would be an "atom." They do not really correspond to TRUE and FALSE. So, I'd suggest just defining them as strings for themselves, e.g. define(INSERT, 'INSERT'). Then, code can check if ($op == INSERT).

* Add a PHPdoc block for each constant explaining what it is for. See field.module for an example. Make sure the PHPdoc is in the same @defgroup as the function(s) that use the constants so they show up on the same page at api.drupal.org.

* Since hook_field_storage_write() always needs to know whether it is an insert or an update, there is probably no benefit to having a default value for the argument. Just make it mandatory and always provide one value or the other.

* In field_sql_storage_field_storage_write(), you changed the $update argument to $op but then did not change the body of the function to use $op instead of $update. Running the unit tests would have revealed this mistake.

Keep it coming! :-)

Hi. Here is my second-round review. Note that I am asking you to make these changes instead of making them myself in order to help you learn how to do core patches, not because I'm trying to be annoying. :-)

* @defgroup field_storage is at the top of field.attach.inc. But these defines there instead of a separate defgroup.

* You are passing FIELD_STORAGE_UPDATE from field_attach_insert(). This turns out to work because it just causes an extra db_delete() that has nothing to delete, but is still wrong.

* If you set a patch to status "Code Needs Review", the testbot will run all core tests with your patch applied for you.

RTBC.

Re-rolled from docroot.