Improve checking of child pages in contrib/amcheck.

This commit eliminates lossiness in check for missing parent downlinks in
B-tree.  Instead of collecting lossy bitmap, we check for missing downlinks
while visiting child pages referenced by downlinks of target level.  We
traverse from previous child page to the subsequent child page by right links.
Intermediate pages are candidates to have lost parent downlinks.

Also this commit introduces matching of child high key to the pivot key of
it's parent.

Discussion: https://postgr.es/m/CAPpHfduoF-c4RhOyOm%3D4-Y367%2B8txq9Q6iM_ty0OYc8si1Abww%40mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Peter Geoghegan

doc/src/sgml/amcheck.sgml

@@ -125,8 +125,7 @@ ORDER BY c.relpages DESC LIMIT 10;
       Optionally, when the <parameter>heapallindexed</parameter>
       argument is <literal>true</literal>, the function verifies the
       presence of all heap tuples that should be found within the
-      index, and that there are no missing downlinks in the index
-      structure.  When the optional <parameter>rootdescend</parameter>
+      index.  When the optional <parameter>rootdescend</parameter>
       argument is <literal>true</literal>, verification re-finds
       tuples on the leaf level by performing a new search from the
       root page for each tuple.  The checks that can be performed by
@@ -136,7 +135,8 @@ ORDER BY c.relpages DESC LIMIT 10;
       a more thorough variant of <function>bt_index_check</function>:
       unlike <function>bt_index_check</function>,
       <function>bt_index_parent_check</function> also checks
-      invariants that span parent/child relationships.
+      invariants that span parent/child relationships, including checking
+      that there are no missing downlinks in the index structure.
       <function>bt_index_parent_check</function> follows the general
       convention of raising an error if it finds a logical
       inconsistency or other problem.