[Bug]: Poor performance opening "Check Online Content" window due to O(N^3) behaviour

[JGRennison]

Version of OpenTTD

master (9c74dc2), however this is not a new bug

Expected result

Blazing fast, silky smooth, positive adjectives performance when opening the check online content window

Actual result

Frame rate decreases dramatically when the list is being populated.

The performance problem seems to be created in ClientNetworkContentSocketHandler::ReverseLookupDependency via ClientNetworkContentSocketHandler::CheckDependencyState.
This seems to be because each received content item calls CheckDependencyState on each previously received item, which calls ReverseLookupDependency which again iterates all previously received items, leading overall to O(N^3) behaviour in the number of content items. See below for approximate measurements.

Steps to reproduce

1. Open the "Check Online Content" window from the main menu (not when in a game).