Revise child-to-root tuple conversion map management.

 Revise child-to-root tuple conversion map management.
 
 Store the tuple conversion map to convert a tuple from a child table's
 format to the root format in a new ri_ChildToRootMap field in
 ResultRelInfo. It is initialized if transition tuple capture for FOR
 STATEMENT triggers or INSERT tuple routing on a partitioned table is
 needed. Previously, ModifyTable kept the maps in the per-subplan
 ModifyTableState->mt_per_subplan_tupconv_maps array, or when tuple
 routing was used, in
 ResultRelInfo->ri_Partitioninfo->pi_PartitionToRootMap. The new field
 replaces both of those.
 
 Now that the child-to-root tuple conversion map is always available in
 ResultRelInfo (when needed), remove the TransitionCaptureState.tcs_map
 field. The callers of Exec*Trigger() functions no longer need to set or
 save it, which is much less confusing and bug-prone. Also, as a future
 optimization, this will allow us to delay creating the map for a given
 result relation until the relation is actually processed during
 execution.
 
 Author: Amit Langote
 Discussion: https://www.postgresql.org/message-id/CA%2BHiwqHtCWLdK-LO%3DNEsvOdHx%2B7yv4mE_zYK0i3BH7dXb-wxog%40mail.gmail.com