Introduce ability to create MD5 of Coherence NamedCaches

[brianoliver]

As a developer sometimes I'd like to compare NamedCaches across clusters. Comparing each entry between clusters can be expensive in terms of CPU, Memory and Network. As a shortcut, instead we could simply create a MD5 of NamedCache contents and then compare those.

[brianoliver]

This is probably best achieved using a custom aggregator, that creates local MD5s and then combines them, say on a partition basis.