Allow hstore columns to be represented as ImmutableDictionary<string, string>

[roji]

Most of the work here was done by @Quogu in #1184.

This allows using the .NET Immutable type, but also does much faster change tracking since a reference comparison is enough to know whether the dictionary contents have changed.

[roji]

@QuoGo @Quogu can you please post a comment here so I can assign the issue to you? This way you get more visible credit for working on it.

[Quogu]

@roji my username is a tricky one to spell, but I can definitely leave a comment. :) Thanks!

[roji]

Oops sorry :) The important this is that we got there... You can check out #1492 which replaces your PR and is ready to merge, except for needing to depend on a newer Npgsql.