Search through all plural forms of a translation

[Данило Шеган]

As part of https://blueprints.edge.launchpad.net/rosetta/+spec/search-pofiles implementation, I've avoided implementing search through all plural forms of a translation. The code should not be too hard (really similar to existing code for initial form search), but I want to get real performance measurements for searching through one only, and if that proves out to work fine, we can easily extend to search through others as well.

Relevant code in database.pofile.findPOTMsgSetsContaining.

[Данило Шеган (danilo)]

Fixed in RF 6359.