[ASTERIXDB-3955][COMP] Fix CBO cost of a pushed LIMIT

The CBO costed every operator of a plan as if it stopped at a pushed
LIMIT. A limit is pushed only into a primary-index search, so a
secondary-index search still returns every match, and the primary-key
sort after it is blocking. An index plan therefore looked near-constant
and won even when a scan that stops after a few rows was far cheaper.

A primary-index search returns the records itself, so it is no longer
charged a primary-key sort and fetch.

Measured against an index plan, a scan that a limit stops early still
runs several times slower than the rows it reads suggest, so it is
costed as reading eight times further, though never more than a full
scan. Without that weight it displaced index plans that were faster on
selective predicates with large limits.

Ext-ref: MB-70486
Change-Id: Id437d35c7c3e3f9624076464d196518da823400c
Reviewed-on: https://asterix-gerrit.ics.uci.edu/c/asterixdb/+/22002
Reviewed-by: Rithwik Koul <rithwik.koul@couchbase.com>
Integration-Tests: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Reviewed-by: Ali Alsuliman <ali.al.solaiman@gmail.com>
Tested-by: Jenkins <jenkins@fulliautomatix.ics.uci.edu>