revises the documentation of AR::Base.cache_timestamp_format [ci skip]
moves the new :nsec date format to the Active Support changelog [ci skip]
also rename indexes when a table or column is renamed

When a table or a column is renamed related indexes kept their name. This will lead to confusing names. This patch renames related indexes when a column or a table is renamed. Only indexes with names generated by rails will be renamed. Indexes with custom names will not be renamed.