Add amcheck verification of heap relations belonging to btree indexes.

Add amcheck verification of heap relations belonging to btree indexes.

Add a new, optional, capability to bt_index_check() and
bt_index_parent_check():  check that each heap tuple that should have an
index entry does in fact have one.  The extra checking is performed at
the end of the existing nbtree checks.

This is implemented by using a Bloom filter data structure.  The
implementation performs set membership tests within a callback (the same
type of callback that each index AM registers for CREATE INDEX).  The
Bloom filter is populated during the initial index verification scan.

Reusing the CREATE INDEX infrastructure allows the new verification
option to automatically benefit from the heap consistency checks that
CREATE INDEX already performs.  CREATE INDEX does thorough sanity
checking of HOT chains, so the new check actually manages to detect
problems in heap-only tuples.

Author: Peter Geoghegan
Reviewed-By: Pavan Deolasee, Andres Freund
Discussion: https://postgr.es/m/CAH2-Wzm5VmG7cu1N-H=nnS57wZThoSDQU+F5dewx3o84M+jY=g@mail.gmail.com