Fix migrations for all DB providers.

[bitbound]

The Postgres provider was broken due to changes in how DateTimeOffset is handled (in version 6). The SQL Server provider was broken due to multiple cascade delete paths. This PR fixes that.

• Added Remove-Migration and Update-Database utility scripts.
• Added PostgresDateTimeOffsetConverter to make Postgres happy.
• Changes cascades to client-side where needed to make SQL Server happy.
• Added Apply extension method to IQueryable<T> to make it more fluent.

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