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