Fix disable_prepared_statements clobbering pool size

What does this MR do?

In gitlab-com/gl-infra/production#5238 (comment 637008856) we found the pool size reverted back to 5. It turns out that disable_prepared_statements was using a cached value.

This fixes to use the actual db config right before we make another establish_connection

• gitlab-com/gl-infra/production#5238 (comment 637008856)
• !65262 (merged)

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Remove pool from your config/database.yml. Setup load_balancing in config/database.yml.

development:
  host: ...
  # ...
  load_balancing:
    hosts:
      - <same as hosts>
      - <same as hosts>

Test out the main connection's pool size with:

ActiveRecord::Base.connection.pool.stat

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