Update gitlab:db:reindex task and add database specific tasks,
dynamically generated for each configured database.

gitlab:db:reindex      # reindex all defined databases
gitlab:db:reindex:ci   # reindex ci database
gitlab:db:reindex:main # reindex main database

Update exclusive lease keys to include connection name so that multiple
databases can be reindexed concurrently.

https://gitlab.com/gitlab-org/gitlab/-/issues/344235