core: Fix Postgres messagesNewerThan wrong buffer
Make the inner SELECT statement use the buffer ID for bufferid, NOT
the messageid.
This fixes select_messagesNewerThan using the wrong variable for the
buffer ID, causing more recent messages to not show up when fetching
backlog with the "Unread messages" method.
Fixes regression in pull request #273, missed by pull request #274.
NOTE: The client still needs fixed to stop sending invalid IDs. That
should be addressed with another set of commits.
See https://github.com/quassel/quassel/pull/274#issuecomment-
284205802