amcheck: fix multiple problems with TOAST pointer validation

amcheck: fix multiple problems with TOAST pointer validation

First, don't perform database access while holding a buffer lock.
When checking a heap, we can validate that TOAST pointers are sane by
performing a scan on the TOAST index and looking up the chunks that
correspond to each value ID that appears in a TOAST poiner in the main
table. But, to do that while holding a buffer lock at least risks
causing other backends to wait uninterruptibly, and probably can cause
undetected and uninterruptible deadlocks.  So, instead, make a list of
checks to perform while holding the lock, and then perform the checks
after releasing it.

Second, adjust things so that we don't try to follow TOAST pointers
for tuples that are already eligible to be pruned. The TOAST tuples
become eligible for pruning at the same time that the main tuple does,
so trying to check them may lead to spurious reports of corruption,
as observed in the buildfarm. The necessary infrastructure to decide
whether or not the tuple being checked is prunable was added by
commit 3b6c1259f9ca8e21860aaf24ec6735a8e5598ea0, but it wasn't
actually used for its intended purpose prior to this patch.

Mark Dilger, adjusted by me to avoid a memory leak.

Discussion: http://postgr.es/m/AC5479E4-6321-473D-AC92-5EC36299FBC2@enterprisedb.com