PostgreSQL read_stream_backwards is incorrect for backwards reads

[nmummau]

The issue fixed here for SQL Server also has the same bug in Postgres: 73c56ce

Should be a simple fix but needs tested by someone with a Postgres setup, so I didn't want to fix it alongside all of my SQL Server fixes in PR #431

postgresql code function __schema__.read_stream_backwards:

if _current_version < _from_position + _count then
    return;
end if;

This guard is incorrect for backwards reads. It mixes the page size (_count) with the valid start position check. Example: if _current_version = 10, _from_position = 10, and _count = 3, there are clearly events at positions 10, 9, 8. Yet 10 < 10 + 3 is true, so the procedure returns nothing even though valid data exists.

[linear]

EVE-204