Support migrate_last_imported feature from migrate_tools

Problem/Motivation

The migrate_tools module provides a way for storing and displaying timestamp of most recent migration run.
This is particularly useful with `drush ms` command output which has such column.

Unfortunately, migrate_cron doesn't update these values and thus `drush ms` output looks confusing as it displays the last manual migration execution timestamp.

Steps to reproduce

1. Install migrate_tools and migrate_cron.
2. Execute migration with migrate_cron.
3. Run `drush ms`.
4. Observe the 'Last Imported' column from it's output.

Expected: the Last Imported column value should update after cron run.
Actual: the value remains same and only updates if migration is executed with `drush mim` command.

Proposed resolution

Add a submodule for better integration with migrate_tools.

Remaining tasks

Do it.

User interface changes

None.

API changes

None.

Data model changes

None.