Update Gitlab::Database::Count to work with decomposed database

What does this MR do and why?

When counting records with Gitlab::Database::Count split models by database so that we can wrap usage of PgClass in Gitlab::Database::SharedModel.using_connection using the correct connection.

Also use model.connection instead of ActiveRecord::Base.connection.

Related to #345279 (closed).

How to set up and validate locally

1. Setup your environment for multiple databases as described in https://docs.gitlab.com/ee/development/database/multiple_databases.html#configdatabaseyml.
2. Load the schema, e.g. gdk psql -d my_ci_database < db/structure.sql.
3. Connect to the ci database - gdk psql -d my_ci_database.
4. Create some ci_instance_variables records:

   insert into ci_instance_variables values (1, 1, false, false, 'TEST_DECOMPOSED_1', null, null), (2, 1, false, false, 'TEST_DECOMPOSED_2', null, null);

5. Analyze ci_instance_variables - analyze ci_instance_variables;
   1. @alipniagov : also run similar analyze on your primary DB, so pg_class would have data.
6. While on master open Rails console with GITLAB_USE_MODEL_LOAD_BALANCING=true bundle exec rails console
7. Check counts with Gitlab::Database::Count.approximate_counts([Ci::InstanceVariable], strategies: [Gitlab::Database::Count::ReltuplesCountStrategy]), you should see 0 as a count.
8. Switch to this branch - git checkout 345279-decomposed-db-count
9. open Rails console with GITLAB_USE_MODEL_LOAD_BALANCING=true bundle exec rails console
10. Check counts with Gitlab::Database::Count.approximate_counts([Ci::InstanceVariable], strategies: [Gitlab::Database::Count::ReltuplesCountStrategy]), this time you should see 2 as a count.

MR acceptance checklist

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