* context: #1716https://github.com/DapperLib/Dapper/issues/1716
- change type-map to allow nullable
- change DateTime/TimeSpan to null
- do not specify DbType for null
- semi-breaking change: swap GetDbType to SetDbType, noting that it is marked [Obsolete] and internal-use-only
- semi-breaking change: make LookupDbType return nullable (same internal-use/[Obsolete])
* add test for Npgsql 6.0.0-rc.1
* try list parameters (like arrays)
* never assign -1 (sentinel) as the .DbType
* postgres tests