.Net MEVD: Reexamine PostgreSQL timestamp mapping

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The PostgreSQL connector currently maps DateTime to the timestamp without time zone type, and DateTimeOffset to the timestamp with time zone (link).

First, Npgsql itself maps DateTime to timestamp without time zone only for DateTime with Kind=Unspecified or Local, and to timestamp with time zone for Kind=Utc (note that timestamp with time zone simply represents a UTC timestamp in PG, just like DateTime with Kind=Utc).

Second, Npgsql does not support DateTimeOffset with offsets that aren't zero, since there isn't any PostgreSQL type that actually stores an offset in the database (again, timestamp with time zone only stores a timestamp that's meant to represent a UTC timestamp by convention). We should make sure the MEVD connector throws properly for non-zero-offset DateTimeOffsets etc.