Increase ecto pool size for running migrations

[blatyo]

closes #2256

This will fix the immediate issue. In addition, Ecto should probably warn when the pool size is too small or capture the connection checkout error and suggest increasing the pool size.

[fishcakez]

I understand that we need to default to at least 2 connection processes because we are using at least 2 connections, but why are we not using just 1?

[josevalim]

@fishcakez it is because we need one transaction to hold the lock and another transaction to run the migrations. The reason we cannot do everything in one transaction is because some commands like ALTER TYPE cannot be run on a transaction, so the migration needs the ability to control what happens in a transaction and what does not.

[josevalim]

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