Fix segmentation fault in MergeInheritedAttribute()

Split some code out from MergeAttributes()

MergeAttributes code deduplication

Add a const decoration

MergeAttributes: convert pg_attribute back to ColumnDef before comparing

Use extensible buffers to assemble command lines

Clean up command argument assembly

Take pg_attribute out of VacAttrStats

Change type of pg_statistic_ext.stxstattarget

Add macro for maximum statistics target