Remove logs in fetchHeaderBlocks for issue #216

[whunmr]

• remove logs in fetchHeaderBlocks
• extend fetch loop timer to 20s

logs before fix:

root@server:~/.bitcoincash/testnet# du -sh *
4.0K    banpeers.json
11G blocks
927M    chainstate
24G logs

2018/12/10 09:41:00.393 [I] [syncmanager.go:685]  peer(2177) best known block nil, forgive temporary
2018/12/10 09:41:00.393 [D] [chain.go:410]  GetLocator from hash : 0000000000006092cf364fd6d3812013e22ce7f25dd1f8934a0da7e46339c9d2, height : 1273259 .
2018/12/10 09:41:00.393 [D] [syncmanager.go:761]  fetchHeaderBlocks can not find block hashes to fetch from peer(2177)
2018/12/10 09:41:00.393 [I] [syncmanager.go:766]  peer(22) ChainWork less than minChainWork, do not use this peer
2018/12/10 09:41:00.393 [I] [syncmanager.go:685]  peer(771) best known block nil, forgive temporary
2018/12/10 09:41:00.393 [D] [chain.go:410]  GetLocator from hash : 0000000000006092cf364fd6d3812013e22ce7f25dd1f8934a0da7e46339c9d2, height : 1273259 .
2018/12/10 09:41:00.393 [D] [syncmanager.go:761]  fetchHeaderBlocks can not find block hashes to fetch from peer(771)
2018/12/10 09:41:00.393 [I] [syncmanager.go:685]  peer(855) best known block nil, forgive temporary
2018/12/10 09:41:00.393 [D] [chain.go:410]  GetLocator from hash : 0000000000006092cf364fd6d3812013e22ce7f25dd1f8934a0da7e46339c9d2, height : 1273259 .
2018/12/10 09:41:00.393 [D] [syncmanager.go:761]  fetchHeaderBlocks can not find block hashes to fetch from peer(855)
2018/12/10 09:41:00.393 [I] [syncmanager.go:685]  peer(1306) best known block nil, forgive temporary
2018/12/10 09:41:00.393 [D] [chain.go:410]  GetLocator from hash : 0000000000006092cf364fd6d3812013e22ce7f25dd1f8934a0da7e46339c9d2, height : 1273259 .
2018/12/10 09:41:00.393 [D] [syncmanager.go:761]  fetchHeaderBlocks can not find block hashes to fetch from peer(1306)
2018/12/10 09:41:00.393 [I] [syncmanager.go:685]  peer(1405) best known block nil, forgive temporary