Fix cost estimation in CStoreGetForeignPaths.

Previously we also counted in standard attributes like tableoid, cmax, xmax, ... in CStoreGetForeignPaths, which resulted in under estimating the disk access cost. This change fixes that.
Use RelationGetNumberOfAttributes to get number of columns in CStoreGetForeignPaths.
Merge remote-tracking branch 'origin/fix_cost_estimation' into develop