Use Collection instead of Set

Currently the xManyCommand commands hold the keys in a set internally. During marshalling and splitting the set (when the command goes remote), we use HashSet. However, this has drawbacks:
a) if the set passed to command uses different means of equality than Object.hashCode() and Object.equals() we may lose some keys in the derived set - we don't inject dataContainer's keyEquivalence to the commands
b) as these sets are usually just iterated through (contains() is rare) it's less effective to use complex data structures as HashSet - plain ArrayList is sufficient.