Marked the old versions of PrepareFor* that do not use tokens as
deprecated and moved all of the code to use the new versions that
require a token. This makes the scope of the execution object more
explicit so that it will be kept while in use and can potentially be
reclaimed afterward.
Benchmarking in VTK showed significant overhead in the computation
of the reverse connectivity calculation in
ConnectivityExplicitInternals::ComputeCellToPointConnectivity.
This patch adds a ReverseConnectivityBuilder that reduces the amount of
time and memory needed to build the table by using an atomic histogram
approach that avoids a costly radix SortByKey.
Key operations in the new helper class are templated to allow this
approach to be reused by VTK-specific cell array converters.
