pgsql: tableam: Move heap specific logic from estimate_rel_size belowt

tableam: Move heap specific logic from estimate_rel_size below tableam.

This just moves the table/matview[/toast] determination of relation
size to a callback, and uses a copy of the existing logic to implement
that callback for heap.

It probably would make sense to also move the index specific logic
into a callback, so the metapage handling (and probably more) can be
index specific. But that's a separate task.

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/696d78469f3714a8159f7a145af1fe9179fe3291

Modified Files
--------------
src/backend/access/heap/heapam_handler.c | 113 +++++++++++++++++++++++++++++++
src/backend/optimizer/util/plancat.c     |  69 +++++++------------
src/include/access/tableam.h             |  35 ++++++++++
src/include/optimizer/plancat.h          |   1 +
4 files changed, 174 insertions(+), 44 deletions(-)