Pin unread rooms should consider timestamp within each segment

[turt2live]

It sorts by timestamp after falling through, however it should consider the timestamp in each section as well

[turt2live]

note: this was expected behaviour, but failure to math meant it didn't land in the PR.

See matrix-org/matrix-react-sdk#1936

[turt2live]

Fixed by matrix-org/matrix-react-sdk#2265