core: Drop the 'update_lastmsgid' statements

core: Drop the 'update_lastmsgid' statements

Remove the 'update_lastmsgid.sql' queries from PostgreSQL and SQLite
and remove the 'setBufferLastMsgId()' functions from
PostgreSqlStorage and SqliteStorage.  These are no longer needed now
that database triggers handle updating the 'lastmsgid' for buffers.

This removes redundancy and might improve performance.

Resolves GH-290.