Implement keep alive requests for the DB Connection

Description

In my company we have the database on a seperate server and a firewall between the gitlab server and the DB server.
Our firewall cut's the connection after a certain time of inactivity (1 hour by default).
This leads to sidekiq suddenly being unable to query the database, which in turn freezes all sidekiq jobs.

Proposal

To counter this, I propose to implement an optional keep alive query that's being executed every x minutes. I propose to add 2 new config options. (I know, I know dreaded words)

• keep_alive_query (I usually use Select 1, maybe we can also simply hardcode that.)
• keep_alive_frequency (set it in minutes)

At least the keep_alive_frequency would have to be configurable, because the timeout probably isn't the same for all customers.

Links / references

related to ##23755 (closed)
Zendesk Ticket: https://gitlab.zendesk.com/agent/tickets/45666