Add initial-headers-sync timeout handling to fix issue #217

[whunmr]

• Add initial-headers-sync timeout handling to fix issue #217
  • initial-headers-sync Timeout = base + per_header * (expected number of headers)
  • testing log:

2018/12/14 09:46:23.789 [E] [syncmanager.go:237]  disconnect timeout sync peer:213.227.140.194:8333
2018/12/14 09:46:23.789 [E] [syncmanager.go:243]  sync headers from peer timeout, restart sync

• Remove nil handling of GetIndexBestHeader

  • The bestHeader will be set during lchain.InitGenesisChain(), so after appInitMain the bestHeader will at least be GenesisBlock

• Use lightweight IsInitialBlockDownload avoid chain tip access data race

[whunmr]

@whitefen @Radar3 please help to review these commits. thanks.

[Radar3]

I have reviewed the code. Every change looks well. But one thing should be care: If there are very few peers, or only one peer, disconnect that peer may be dangerous.