Fixes to ForwardsOnlyRow

Index management logic on ForwardsOnlyRow was complicated and triggered
two bugs (see tests). Redid logic in a simpler way.
(cherry picked from commit 8c996c849f78e0b156e1a2cf2889b6ef97f57b87)

Conflicts:

	Npgsql/Npgsql/NpgsqlAsciiRow.cs
	Npgsql/Npgsql/NpgsqlRow.cs