rlp: encode nil array pointers as empty list or string

XEth cleanup

bugfix with idle too long triggered after peer switch

* In case of uncle mining (e.g. same TS) the miner would stop if all
  threads happened to mine a potential uncle

- reset bestIdleTimer when launching head section process
- log chain head events
- common.Hash key in activateChain map

Update ethash lib

* Refund of value