Remove Sqlg's topology lock.

[pietermartin]

This ticket arose out of #329

Look at removing all locks from Sqlg and let postgres throw dead lock exceptions rather.

Removing the topology lock needs careful thought and testing as it puts the burden on the client to retry failed transaction due to postgres dead lock exceptions.
This will also most likely break existing applications so maybe make it optional to use Sqlg's locks or not.

[pietermartin]

To clarify the burden on the client. During concurrent schema creation the database will throw exceptions. In postgresql it is table already exist or similar exceptions. In this case the client will have to rollback and retry the transaction.