Terminate sortkey with prejudice

It has not been used by the search system since 2.8 and all code using
it in a meaningful way in Gerrit has been removed. Now we can remove
it from the database and API classes.

This requires a schema upgrade to delete the column and rebuild some
indexes to use last_updated_on instead of sort_key.

Change-Id: I87660f8e0e44bf11bafd9fa24f990351759df3fb
25 files changed