Allow PostgreSQL adapter tcp parameter tuning

What does this MR do?

Allows administrators to tune the TCP connection parameters in the PostgreSQL adapter for Rails.

Closes: #5634 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened

Closes #5634 (closed)

Testing

1. Install package and minimal gitlab.rb See dev builds
2. Run gitlab-ctl reconfigure
3. Take copy of /opt/gitlab/embedded/service/gitlab-rails/config/database.yml

   cp /opt/gitlab/embedded/service/gitlab-rails/config/database.yml ~/4776_original_database.yml

4. Add following to /etc/gitlab/gitlab.rb

   # other settings...
   gitlab_rails['db_keepalives'] = 1
   gitlab_rails['db_keepalives_idle'] = 5
   gitlab_rails['db_keepalives_interval'] = 3
   gitlab_rails['db_keepalives_count'] = 3
   gitlab_rails['db_tcp_user_timeout'] = 13000

5. Run gitlab-ctl reconfigure
6. Validate diff between copy and new database.yml

   diff /opt/gitlab/embedded/service/gitlab-rails/config/database.yml ~/4776_original_database.yml

7. Expect diff to look similar to:

   23,27c23,27
   <   keepalives: 1
   <   keepalives_idle: 5
   <   keepalives_interval: 3
   <   keepalives_count: 3
   <   tcp_user_timeout: 13000
   ---
   >   keepalives:
   >   keepalives_idle:
   >   keepalives_interval:
   >   keepalives_count:
   >   tcp_user_timeout:

8. Clean up

   rm ~/4776_original_database.yml