pgsql: Consolidate VACUUM xid cutoff logic.

Consolidate VACUUM xid cutoff logic.

Push the logic for determining whether or not a VACUUM operation will be
aggressive down into vacuum_set_xid_limits().  This makes the function's
signature significantly simpler, and seems clearer overall.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WzkymFbz6D_vL+jmqSn_5q1wsFvFrE+37yLgL_Rkfd6Gzg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/efa4a9462a072b4cd6459058df274dd23022d0e2

Modified Files
--------------
src/backend/access/heap/vacuumlazy.c |  34 +++++----
src/backend/commands/cluster.c       |   3 +-
src/backend/commands/vacuum.c        | 131 ++++++++++++++++-------------------
src/include/commands/vacuum.h        |   6 +-
4 files changed, 79 insertions(+), 95 deletions(-)