This extends cluster_rel() in such a way that more options can be added
in the future, which will reduce the amount of chunk code for an
upcoming SKIP_LOCKED aimed for VACUUM.  As VACUUM FULL is a different
flavor of CLUSTER, we want to make that extensible to ease integration.

This only reworks the API and its callers, without providing anything
user-facing.  Two options are present now: verbose mode and relation
recheck when doing the cluster command work across multiple
transactions.  This could be used as well as a base to extend the
grammar of CLUSTER later on.

Author: Michael Paquier
Reviewed-by: Nathan Bossart
Discussion: https://postgr.es/m/20180723031058.GE2854@paquier.xyz