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Starts any connection pooling or supervision and return `{:ok, pid}`
or just `:ok` if nothing needs to be done.

Returns `{:error, {:already_started, pid}}` if the repo already
started or `{:error, term}` in case anything else goes wrong.

## Adapter start

Because some Ecto tasks like migration may run without starting
the parent application, it is recommended that start_link in
adapters make sure the adapter application is started by calling
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