Npgsql 4.* support and bug fixes

Full list of bug fixes and changes:
https://github.com/JasperFx/marten/milestone/26?closed=1

Read more:
https://jeremydmiller.com/2018/09/27/marten-3-0-is-released-and-introducing-the-new-core-team/

Restrict Npgsql version >=3.2.5 and <4.0

https://github.com/JasperFx/marten/milestone/29?closed=1

Bug fixes and more

https://github.com/JasperFx/marten/milestone/27?closed=1

2.9.0

https://github.com/JasperFx/marten/milestone/24?closed=1

2.8.0

See https://github.com/JasperFx/marten/milestone/23?closed=1

v2.7.1

Fix for #987

Bug fixes and improvements to ViewProjection

See https://github.com/JasperFx/marten/milestone/22?closed=1 for more information

2.6.0

Lots and lots of bug fixes and new convenience methods for event sourcing projections. See https://github.com/JasperFx/marten/milestone/21?closed=1

2.4.0

• IDocumentSession.Eject()
• Ability to create a session with an existing connection, transaction, or enlist in ambient TransactionScope
• Feature schema extensions

2.3.2 Bug fix release

• Using fields in compiled query objects
• Bug where it was erroneously checking the existence of the event store schema every time
• Permutation bug w/ FK's + subclasses