date_update_7007 will fail with different field storage backends.

Problem/Motivation

date_update_7007 loops over all datestamp fields and unconditionally calls db_change_field on some tables. On sites using the core provided Field SQL Storage backend, this is fine and will work fine. If you have configured a different field storage backend and are using it for datestamp fields, then the update hook will fail.

Steps to reproduce

Use a different field storage backend that doesn't create the same tables as the core field_sql_storage backend, have a datestamp field, and run the update hook.

Proposed resolution

You can't cater for all the field storage backends out there, which is why the API for doing these field schema changes never made it into D7 core properly. So instead, we should cater for the 99.9% of people using field_sql_storage, but not break for the other 0.1%.

Add a db_table_exists call around the existing code.

Remaining tasks

Code the patch.