GitLab

Rebalancing issues in a long running transaction with a lock
retry generates subtransactions that lead to overall DB
performance degradations.

see https://gitlab.com/gitlab-org/gitlab/-/issues/338346

So we are moving issues rebalancing out of one single big
transaction by locking the repositioning within a project or
namespace during the rebalance.

Rebalancing is a long running job, it can take multiple hours
to finish a rebalance of a namespace with large number of issues.
This change introduces a simple mechanism of resuming rebalance
from a checkpoint.

- A limited number of rebalancing jobs are allowed to run,
5 at this point
- Before starting a rebalance the number of running rebalances
is checked.
- If the limit of running rebalances is met we store the first
project id from the list of projects to be rebalanced, use that
to determine how many rebalances are running
- We load all namespace issue ids into a redis sorted set, by
using current i...