Fix lastseenmsgid updates in PostgreSQL

GH-273 introduced a workaround for the issue where sometimes the
lastseenmsgid for a buffer was set to a msgid not in that buffer.
However, the workaround was incorrect as the "min()" function in
PostgreSQL is an aggregate function for use on a column, not a
function to return the least of a list of values passed in.  The
correct function for use here is "least()".