Migrate shared runner minutes to separate table

This MR fixes the problem with resetting the quota, but this solution is suboptimal: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2275.

We should create a new table which would hold runner quotas (for groups and jobs) per-month and instead of "deleteing"/"resetting" just append a new data to the table.

Proposal

Create two tables:

group_shared_runner_minutes:
 - group_id: integer
 - year: integer,
 - month: integer,
 - minutes: integer

project_shared_runner_minutes:
 - project_id: integer
 - year: integer,
 - month: integer,
 - minutes: integer

We would key the group minutes by: group_id, year, month of time when we check the limits.

We should have an index on project_id and project_id+year+month.

We should add a cron job run monthly that would evict data older than 12 months. Effectively it is the change of ClearSharedRunnerMinutesWorker behaviour.