Store repo in pool options

[v0idpwn]

Useful as options are emitted with :db_connection telemetry events.

This would allow us to correlate telemetry events with the pool owner repo with ease.

Related to discussion in elixir-ecto/db_connection#251

[v0idpwn]

Telemetry metadata with this change:

  metadata: %{
    error: %DBConnection.ConnectionError{
      message: "connection not available and request was dropped from queue after 2265ms. This means requests are coming in and your connection pool cannot serve them fast enough. You can address this by:\n\n  1. Ensuring your database is available and that you can connect to it\n  2. Tracking down slow queries and making sure they are running fast enough\n  3. Increasing the pool_size (albeit it increases resource consumption)\n  4. Allowing requests to wait longer by increasing :queue_target and :queue_interval\n\nSee DBConnection.start_link/2 for more information\n",
      reason: :queue_timeout,
      severity: :error
    },
    opts: [
      log: #Function<14.35606316/1 in Ecto.Adapters.SQL.with_log/3>,
      source: "my_schema",
      repo: EctoBug.RepoWithInit,
      timeout: 15000,
      pool_size: 10,
      pool: DBConnection.ConnectionPool
    ]
  }

[v0idpwn]

Not 100% sure if this is desirable tho :))