Support reindexing multiple databases independently and concurrently

What does this MR do and why?

This MR updates gitlab:db:reindex and adds 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

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

Related to #344235 (closed).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.