Add progress reporting for CLUSTER and VACUUM FULL.

Add progress reporting for CLUSTER and VACUUM FULL.

This uses the same progress reporting infrastructure added in commit
c16dc1aca5e01e6acaadfcf38f5fc964a381dc62 and extends it to these
additional cases.  We lack the ability to track the internal progress
of sorts and index builds so the information reported is
coarse-grained for some parts of the operation, but it still seems
like a significant improvement over having nothing at all.

Tatsuro Yamada, reviewed by Thomas Munro, Masahiko Sawada, Michael
Paquier, Jeff Janes, Alvaro Herrera, Rafia Sabih, and by me.  A fair
amount of polishing also by me.

Discussion: http://postgr.es/m/59A77072.3090401@lab.ntt.co.jp