Split PGPROC 'links' field into two, for clarity

Split PGPROC 'links' field into two, for clarity

The field was mainly used for the position in a LOCK's wait queue, but
also as the position in a the freelist when the PGPROC entry was not
in use. The reuse saves some memory at the expense of readability,
which seems like a bad tradeoff. If we wanted to make the struct
smaller there's other things we could do, but we're actually just
discussing adding padding to the struct for performance reasons.

Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://www.postgresql.org/message-id/3dd6f70c-b94d-4428-8e75-74a7136396be@iki.fi