Revert "Recycle NpgsqlTransaction" (#2416)

[roji]

This reverts commit 7b8e117 (issue #2416) for 4.1, which made us recycle transaction instances on a connector.

• As part of transaction recycling, NpgsqlTransaction.IsCompleted was made internal, since the transaction instance will be revived, IsCompleted will switch from true to false, leading to potential confusion/dangerous behavior. This was a problem for at least one user ([4.1.0] experience report updating Marten #2607).
• Even with transaction recycling, it's still possible to expose an IsCompleted property that would only be valid until the instance is disposed. However, currently there's no requirement to dispose NpgsqlTransaction - the moment a transaction is committed or rolled back, a new one can be started again. Changing that would be an even bigger breaking change.

Note that the commit remains in dev for 5.0, where we should decide on the exact behavior we want, opened #2618 to track.