Add lock_id and locked_by to terraform state model

Add locked_at column to terraform states table

Add terraform state name with proper locking columns

Add first version of terraform state handler service

This adds initial version of Terraform state handler that supports
locking, and is going to be used in our Terraform Remote Backend state
API implementation.

Update terraform state migrations

Improve terraform state handler and related specs

Rename terraform lock id column to lock_xid

The rename is consistent with our recent guidelines to avoid using `_id`
suffixes for things that are not relations. It is recommended to use
`_xid` is such situations.