Rename epics state column to state_id

Rename state column to state_id in epics table.

Epics table have an integer column called state. We are changing issues and merge requests table to also have an integer column called state_id. To be consistent we should have all columns named as state_id

For more context check: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25107#note_155152188