Rename message_id to time_id. Use _id for sequential message index given by producer and time_id for message based on arrival time.

Add optional argument `ordered` to consumer function get_next. If true messages will be ordered by message_id otherwise by _id.

This adjusts the paths of the pkg-config files so that they match
the installation locations of the actual libraries (always in
prefix/lib, not prefix/lib64).  It also fixes the paths within the
pkg-config files themselves, which were incorrect.