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ARROW-6157: [C++] Array data validation

Add a method ValidateFull() on arrays, batches etc. which does O(N) data validation
for a few types (list, union, dictionary).

Also, fix the assumptions about union arrays to match official semantics.

Closes #5892 from pitrou/ARROW-6157-array-data-validation and squashes the following commits:

c8983f643 <Antoine Pitrou> ARROW-6157:  Array data validation

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>