* Reduce Nim 1.6 compiler warnings/hints for Fluffy and Nimbus proxy
Mostly raises Defect removals, TaintedString removal and some
unnecessary imports.
Also updating the copyright years alongside.
* Further reduce Nim 1.6 compiler warnings/hints for Nimbus
- Move the accumulator definitions to a history accumulator file
- Add accumulator build helper calls + temporary database
- Add a header gossip content key encoding test
- Refactor & some cleanup