[FAB-2196] Improve subset efficiency
[FAB-2196] Improve subset efficiency

https://jira.hyperledger.org/browse/FAB-2196

Per feedback from @muralisr in

https://gerrit.hyperledger.org/r/#/c/5771/

It makes more sense to check for set size before iterating to check
subsetting.  This CR implements that suggestion.

Change-Id: I82cccb35ce8e4982d12d354baed3f9f8babea8d8
Signed-off-by: Jason Yellick <3a1724207862302656e55e6f840a9996dad69e59@us.ibm.com>

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