Add missing and dangling downlink checks to amcheck

When bt_index_parent_check() is called with the heapallindexed option,
allocate a second Bloom filter to fingerprint block numbers that appear
in the downlinks of internal pages.  Use Bloom filter probes when
walking the B-Tree to detect missing downlinks.  This can detect subtle
problems with page deletion/VACUUM, such as corruption caused by the bug
just fixed in commit 6db4b499.

The downlink Bloom filter is bound in size by work_mem.  Its optimal
size is typically far smaller than that of the regular heapallindexed
Bloom filter, especially when the index has high fan-out.

Author: Peter Geoghegan
Reviewer: Teodor Sigaev
Discussion: https://postgr.es/m/CAH2-WznUzY4fWTjm1tBB3JpVz8cCfz7k_qVp5BhuPyhivmWJFg@mail.gmail.com

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