Investigate local slow Ecto connection

[achiurizo]

Have not seen this on CI yet but locally:

1) test changeset/2 encodes the output_id (Engine.DB.OutputTest)
    apps/engine/test/engine/db/output_test.exs:25
    ** (DBConnection.ConnectionError) connection not available and request was dropped from queue after 162ms. This means requests are coming in and your connection pool cannot serve them fast enough. You can address this by:
....
      1. By tracking down slow queries and making sure they are running fast enough
      2. Increasing the pool_size (albeit it increases resource consumption)
      3. Allow requests to wait longer by increasing :queue_target and :queue_interval
....
    See DBConnection.start_link/2 for more information

Originally posted by @achiurizo in #69

Need to check out what the issue is, and if we can remove these configurations.

[InoMurko]

We should do this: omgnetwork/elixir-omg#1689