Improve performance of PostgreSQL for large databases.

This change requires a change to the database schema.

Currently, in cases where buffers exist that have not had any activity for an extended period of time, during initial backlog fetching, Quassel will have to scan backwards through all activity in all buffers until it reaches activity for the buffer in question.

This adds an additional field to the buffer table, "lastmsgid", which is the messageid of the most recent message added for that buffer. This improves performance by allowing Quassel to skip straight to where the most recent activity was, instead of scanning millions of rows (in large databases) in futility.