Lightweight address iterators in Skycoin plugin

[olemis]

Is your feature request related to a problem? Please describe. Every time address are being enumerated they are copied onto a slice. This has a negative impact upon memory allocation.

Describe the solution you'd like Implement lightweight iterators reusing the actual collections and container objects including addresses.

Specific tasks have to be written down once a deeper review will be made.

Additional context Discovered while reviewing #264 for #231