Lightweight address iterators in Skycoin plugin

**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